Sometimes, no matter how peaceful your intentions, you'll come across someone (or something) that just will not let things lie. It might be an evil spirit, it might be another Projector, or a variety of other things. Here's my first piece of advice:
Just walk away
Yes, I know that it can be tempting to go on the offensive, especially in a place where you can't be hurt, or arrested, etc. But believe me, Astral combat usually results in nothing but a waste of time and energy. I advise you to avoid it whenever possible.
But sometimes, just sometimes, that won't be an option. If it's someone who'll hold a grudge, or if they attack you persistently, it'll seem that the only way to STOP them is to fight them off. My advice?
Don't fight them
There are very few things any spirit can do to you, when you get right down to it. They can attack you, yes, but so what? They can't HURT you. They can, if they're skilled, send spells of bad luck or put a curse on you. So learn to use your power to protect yourself. (See Key Figures) They might send you nightmares every night. So learn to ward your dreams against interference. They might make it hard for you to Project, by disturbing you. So ward your room to keep negative spirits out. You don't need to be projecting to do all these things. There are very few things that can't be dealt with by learning to protect yourself.
But, sometimes, for whatever reason, you may be in a situation that you don't think you can resolve any other way. I've been in that situation once or twice. The first time, I got literally chased off the Planes, blocked from re-entering, and sent nightmares every night by a VERY nasty spirit I encountered by trying to help a friend who was having recurring nightmares. There were, tho I didn't know it at the time, other ways I could've gone about it, but I hadn't been projecting all that long. I didn't know that. So, I fought it instead, when I finally made my decision to stand up to it. (And yes, I did defeat it, eventually, but it wasn't easy)
So, if you ever wind up deciding to fight, consider these points:
If you decide to fight, you have to win. Otherwise there's no point in fighting. If you don't think you can, find another way. Most importantly, the last two points should be considerered. There are many situations where winning gains you nothing. If you fight and defeat an opponent, great. What happens then? You can't kill them, after all. As soon as you turn your back, they may attack you again. And if you loose? You'll have taken a serious knock to your confidence in your abilities, especially if you thought you were going to win. And that can be hard to get over.
Really, knowing as much of magic and the planes as I do now, I can tell you this: There is NEVER any situation that NEEDS you to fight someone. You might WANT to, you might think you NEED to, but there is ALWAYS another way. If something's being a pest, avoid it. Create a key figure that'll keep it away. Avoid the planes it usually occupies. If you really want to, go ahead and fight, but don't EVER say you had no other option.
However.
There is ONE valid reason for fighting on the Planes. Practice.
Ever been to a martial arts class? If not, I'm sure you'll have heard of them. You get a lot of people going, and for these basic reasons:
Those are good reasons, in my book. And you'll notice, "Being able to beat people up easily" is NOT part of the list.
Astral Combat is a bad idea. But Astral Competitions are not. The difference? Your opponent is a friend who's agreed to fight you, not an enemy.
Astral Competitions are an excellent training exercise, if you can find a partner. The basic point is to try and force your opponent off the Astral Planes and back to their body, while they do the same. That's about the only rule. After all, you can't get genuinely hurt. And it's an amazingly effective way of learning how to use magic quickly, effectively, and under pressure. Just like playing a two-player game like Doom, Quake or Duke Nukem is much, much harder than playing against the computer monsters, so is Astral Competition much harder than using magic at your own pace. And that means you become BETTER at it. Nothing gives you more incentive to create an Astral construct, move around the planes, track someone down, blend in with the Astral environment, detect hostile intentions etc than knowing that someone else is trying to do the same thing to YOU. Two people doing everything they can to knock each other off the Planes can become better at magic in a week than a solitary practicioner can become in a MONTH.
I will give you ONE piece of advice tho: Try and pick a plane that is fairly uninhabited for the contest. For one thing, it's better manners. For another, the inhabitants might join in the attack, and having LOTS of enemies is a bit overwhelming. And finally, because in the less-occupied planes, you can more easily warp the entire plane into a different setting. Such as instant cracks in the ground to swallow your opponent, or a mountain falling on them. You get the idea.