Um, actually the guard waypoint would do the opposite - send the AI off after any perceived threats anywhere on the island
The Hold waypoint however should do it, if we're talking a whole group. The problem comes when people use AI mods like GroupLink or ECS that "take over" the native AI and send them off on wild goose chases everywhere.
So, to absolutely, positively make an AI stand still, you need to use disableAI->
unitName disableAI "MOVE"; unitName disableAI "TARGET";
These two together (unitname = name of unit, or if you're putting them in the init line,
this will do)
"MOVE" disables regular moving, and "TARGET" disables the "2, engage that, man, at 3 o'clock!"-styled moving. Both are needed AFAIK to make sure the AI doesn't move.
The problem is of course that, I think, the old bug since OFP-times still persists in which a loaded SP game will re-enable all disableAIs (and do other strange things), but if you don't think this is a problem then you'll be fine. Otherwise there're ways to get around that too.
But...maybe a simple Hold waypoint should be enough!
Wolfrug out.