I play regularly on GTA Online with my mates, in the last couple of weeks we almost already encounter cheaters while in a public session. They can do whatever they want which is very annoying (and being always in passive mode to protect ourselves is not funny), so we are asking ourselves if GTA Online is cheat-protected ?
The servers are, yes.
Hackers/Cheaters get banned every ban wave - which seem to be every couple months (unless a serious hack/cheat is found and a hotfix is pushed).
Definitions of hacks/cheats are cloudy, but this article suggests it's 2 things
We’re trying to find out exactly what the ban criteria is, but it seems to be anyone who’s used either a cheat engine or a DLL injector – things like Script Hook and the Field of View mod.
If a hacker/cheater drops you money, I'd be cautious to pick it up, because as the same article says
There’s also some uncomfirmed reports that if you happened to have received $GTA from a modder then you have been banned too, but then some people who did this have been banned, others haven’t
This is an e-mail from Rockstar detailing what will get you banned;
You can never really stop hacks/cheats from being used. This steamcommunity posts explains some things about the ban, but doesn't cite sources, so I'm unsure if they're reliable.