Do I have to worry about cheaters?

I had someone join my server a few weeks back and they were pretty good. I added them to my friends list and forgot about them until last night. I see them pop up, I invite them; all is going well.

All of the sudden they start throwing guns on the ground. I think to myself "ok, maybe they're getting ready to leave and they just are being nice by throwing their money and guns."

But they didn't stop throwing stuff. I pulled up the tab key and their money kept growing with them just standing there, throwing all other 5 party members best in slot weapons on wave 3.

I session banned them and they rejoined before the next wave? Is this something I have to worry about? I don't want to get banned or suspended for having people that cheat in my server. I have the Valve Anti-cheat method enabled. Not sure what else to do except unfriend them. Apparently banning them doesn't stop them from joining...

Thanks,
sathenzar

Do I have to worry about cheaters?