Busy will fix it, but sadly that makes you also unable to receive tells from your friends etc, while under that status. As others have mentioned, you can have them at the bottom of the screen and they will expire after a while, but mostly that doesn't fix the problem. Here is how I handle it, I either put them on my black list, that will stop that non sense, or and will report them just like RMT but not using the RMT function of course, since it could be seen as "wrongful" report. In most cases after somebody reported that person more than once, a warning will be given out, stopping a lot of people from just spamming FC invites all day long, because the FC requirements are to recruit as much people as possible.