You can cancel them if they set it in the future and you see it before it starts. I do this sometimes for opponents that are in the same Cup or CL as I am. They want to practice against my team because they know they will have to get by me at some point. I do my best not to allow that.

When a final is approaching I will actually put up a totally different formation and tactics just in case they try that. I tend to have the best teams in my lvl among my friend list so this issue can pop up.