I think there should be a thread where you can post if you were denied the opportunity to challenge a gym. You can also post if you are having trouble finding the leader on AIM and he/she won't respond at your attempts to schedule a battle through PM or VM. This will make it easier to differentiate leaders who deny challenges often or aren't active from the ones that just don't get challenged. It will also serve as a preventative measure, as some leaders will not want to be posted about.
A single report won't really do anything, but an arbitrary number of reports on a single leader in an arbitrary time span will.