All true but, for the OP, it doesn't help. I too had an issue with a friend who supported against me repeatedly. Ofc there is a possibilty he thought this was just part of the game since we were in the same league and top 2, so I was nice and gave benefit of the doubt and then it occured in CL too. So I kicked him. I then got repeated requests that I could leave to expire or reject. Still got more requests after that.

Turns out the guy changed tactic and started supporting my games and so I accepted the next request I got but the first support for opposition and he's out. It could be he's just trying to draw me into his league for revenge and I wouldn't mind that one bit.

But the point is although you can reject and try to ignore the requests there is no way to stop them occurring afaik and rejecting repeatedly is tedious.