Hi Anj
From your short post, this screams to me of lack of localized supervision. If you are the only supervision for this number in a large building then this is the real problem. Although 12 staff are not normally a problem, they are when they are working 500 yards apart from each other.
If not and you have supervisors or team leaders, then its time to reel them in.
Be careful about threats unless you can carry them out as people will see straight through them and often interpret them as a lack of experience and then play with you just to let you know they are not stupid.
Be also careful about imposing any system that will simply substitute their inefficiency with another (ie yours) they will love that as well.
Try and find the reason rather then the quick fix which will always come back and bite you one way or another.
If they are doing it simply because they can, then that’s lack of supervision or work.
If they are doing it because they simply don’t care, then that’s attitude and unless you can change it, it wont matter what you do, they will find a way around it. In this case the answer is clear.
Remember never bite and never take anything personally, just analyze and adjust.
Sorry if I have stated the obvious, but without any further details its a bit of a guess.
DP