What can legally be done if I, and others, suspect a security guard of theft at work?

Many thefts, no cameras, and no visual proof this person is stealing, but other facts point to him/her. If the security guard finds out who is making accusations, can he do anything legally about people talking about him/her?

It's not a good idea to make unsubstantiated accusations.

The guard you suspect may in fact be innocent, and if he/she is you are opening yourself up to a law suite.

Perhaps your employer should set up a hidden camera to see if it will reveal who is the actual the thief!

I hate snitches. And I am honest too. Its just that most of them are busi bodies.

Catching him without a camera on premisis is going to be difficult at best. Check his resume. Contact his previous employers and find out if there have been accusations form others.
Oh here's an idea. Just tell him that you're installing 24hr cameras and that his last day is in a week. You can camp out to see if he takes anything. Even if he don't you'll still be rid of him.

suing for slander is always an option if the talk is wrong and he get's canned.

If you go to the bosses and announce that the guard is a thief, you could be in trouble. If you go the bosses and describe the evidence and point out that things are missing and the evidence shows that he had the only opportunity, and the boss draws his own conclusion, then you are fine.