How can I stop invitation spam?

Solution 1:

"Am I able to restrict group invitations so that they only originate from friends?"

The short answer: No.

People have been complaining about this on the Steam forums for over a year. There is also a suggestion for this feature on the Steam forums but alas, Valve in their infinite wisdom have not implemented this feature (hopefully some day they will).

Can I do anything to mitigate the amount of group invites I am receiving?

Yes! You can:

  • Set your Profile Status to "Friends Only"
  • Don't join any groups on steam
  • Don't join any servers
  • Block all communications with a person

I personally only do one of the above, and that is set my profile to "Friends Only". I haven't received any Group invites in a long time since doing that.
Some servers have a plugin to automatically invite you to their group when you join their server. I mostly only play on competitive servers so this hasn't affected me.
If you're finding one particular person is harassing you non-stop with invites, you can Block all communications with that person.

Also,"You shouldn't be invited to a group after declining an invite to it just once."