There will always be that 2% of outright idiots in any demographic. Pilots are not immune to this and there is absolutely nothing you can do about it. In the cruise you are within range of 200/300 aircraft at any time. It is inevitable that once in a while one of those 4/6 idiots is going to do something immature on 121.5
I have said it once, twice, and I'll say it once again: let them be. They know very well that they are on guard, they want you to say 'on guaarrrrrd', it is a small victory for them to be able to reply 'you tooooo', and you really make their day if you start ranting about guard being an international emergency frequency.
Just turn down the volume.