if you know this player is going to be a pain to deal with, it doesn't really matter what their app is like or who they play. you're the mod. you shouldn't need to write an essay on why it's acceptable to not let someone in your game. you just don't want to.

sometimes you just don't want to deal with someone you know is going to be a headache. life is way too damn short to grin and bear it. especially if it's your playroom. no. no headaches allowed.

so i would just decline their app. if they asked for a reason, it really depends on how avoidant you want to be. i personally wouldn't mind telling them that due to their past behavior in other game, i don't feel they'd be a good fit with mine and i didn't want to deal with their behavior. i don't want to have to one two three strike them out. i just don't want to deal with them.