I came on to post the same thing, since I'm apparently not allowed to ask about it ingame. Yes the server was hacked.... again. Same douchebags from TKC........ again, all they are doing is running simple scripts as addons, if you don't allow their addons then they are SOL.
Here's how you do it, for tutorial's sake we will say you want to enable your clients to join with @xxx mod.
1) In the server.cfg, look for and edit or add this line:
2) Download xxx mod from a legitimate site, and look for a file or files that end with .bikey ie "xxx.bikey"
3) Move the bikeys to the "keys" folder in your server directory.
4) Make sure players joining have the bisign files in their addons directories (there are often multiple of these per addon)
Also note: With VOP mod the 2.1 version was released to the major sites with keys, then the author made a small update which he only released on the BIS forums with more keys, make sure you decide which one to use and the bikey and bisigns coorespond to the correct version.
5) Join game with chosen mod and see if it kicks you.
For more server.cfg variables look here: http://community.bistudio.com/wiki/server.cfg
and : http://community.bistudio.com/wiki/ArmA:_Addon_Signatures