People Who Eat Food That's Not For Them

Dervish, we pay for food for our group at a hotel event, but it is often set up in the common area. The table has a sign indicating what meeting it is for (e.g., International Wording Dervish Convention), but people from other meetings think nothing of grabbing a cup of coffee or pastry as they pass by. Some are so bold as to continue even when asked not to. Short of installing a remote fart machine below the table to embarrass them, what can we say?

I wouldn't take food from your house
And help myself to your fine spouse
This spread's reserved
You greedy perve
Desist or your ass I will oust