Honestly, it's easier for me to deal with it before my spot looks like a good one. If someone comes into you with inertia, you may not be able to do anything. If you're stable, you can always countersteer into them a bit to see if you can stop their progress and hold your spot. Make sure contact is with your forearms, and not any bike parts (one good reason to be in your drops when you sense this activity approaching).

To prevent people from wanting the spot, I often half draft and overlap by a full wheel or a half bike length. If you get your hands close to your draft's hip, your front wheel is protected again, and you don't have to worry about the overlap. The draft is well over 50% if you're on the downwind side when doing this, and they can't move you off the wheel because the guy is clearly in your way. It's not like they can force you to back up and then go around.

I'll slide out into this position when I sense that people are starting to want my spot, assuming I want to keep it. Most of the time I just let riders in, but there are certainly times when that's not the case.