How to stop people from hacking on Garry's mod

You can buy yourself an anticheat like CAC, but it's not going to do much but stop a few hacks.
Gmod is sadly a game that's too easy to hack and you can't do much to stop hackers.
You might be able to find yourself a small, relatively unknown and expensive anticheat plugin/addon that'll give you some OK results, though it won't pick up everything.
Gmod needs a better anti cheat. As far as I know it has VAC, but it's not super great.... not that VAC is good at all (lmao csgo).
Most hacks are clientside, and will not be stoppable.