I'm sorry to hear of this latest development, Mensch.
However, someone must take control of the situation.
The facility should be run like a business; in a concert hall you don't have people walking in off the street playing the piano, much less tuning it!
A grand piano is a valuable asset that must be managed, in an environment that should be controlled. Since a church is less likely to be secured, all the more reason to take extra precautions with the instruments.
The first thing I would do, as soon as possible: set up a meeting. The people in charge need to understand the seriousness of the situation (in other words, sieze the moment
and make use of this latest situation to help fix the problem). Then, someone needs to be put in charge of overseeing the instruments. Hopefully this person will have an idea of how much money it costs to replace a string, buff out a scratch, repair a soundboard, etc. A gouge in a polyester finish can cost $500 to repair - that's not insignificant!
Next, you need to come up with a written policy: who may use the instruments, and what they may or may not do with them. It should not be too oppressively restrictive, but should be firm, concise and clear. A copy of this should be given to anyone who rents the facility or uses the instruments; ideally they would sign something to indicate they understand the policy.
Next, you need an "event host" who is present for every event and who has the authority to act upon the policies. If anything is damaged they should immediately document it for future reference.
Although it is a pain, I highly recommend that the more accessible instruments be locked. This is not intended to turn away people who have a legitimate need to access the instruments but is more of a sign of respect; "gee, this has a lock on it - maybe it is valuable"!
I was a very unpopular person in my church after putting a lock on the new piano. However the ritual of unlocking the piano, keeping track of the lock and key, and removing/folding the heavy-duty piano cover seem to have the intended effect: everyone takes extra care of the piano, even if "stewardship" is a foreign concept to them. 
Soubasse32