It isn't just recently, there have been modders ruining the online experience for a long time now. I've reached Rockstar multiple times concerning the issue and they have yet to assure that something is going to be done about it.
For now we can form together groups of individuals who don't modify their Xbox 360.
Just file a complaint on any modder you see, for 'System Tampering'. The more reports these people have, the more chance they have of being banned. I know it works because the majority of modders I see have little to NO gamerscore, meaning that account
is probably just recently created on another non-banned console.