The message is cached on all the instances of meshchat on the network. Unless there's a new feature recently relessed by Trevor. It's probably not an option to manually delete on all meshchat instances--one person is unlikely to have admin access to do the manual surgery on the cached message file on every mesh node (and might need to turn off the syncing everywhere so it doesn't keep propagating the message back just after you delete at a given instance).
If there's a really inappropriate post, I suppose one could enter 500 new messages to flush it out (or whatever the max count is). Although, this is kind of like publicly writing on the chalk board 500 times, "I will not post messages I later regret" :) .
After making sure mesh chat wasn't open anywhere, I was able to log into each node as root via ssh and rm /tmp/meshchat/messages.MeshChat. Not the best way probably, but it worked.
If there's a really inappropriate post, I suppose one could enter 500 new messages to flush it out (or whatever the max count is). Although, this is kind of like publicly writing on the chalk board 500 times, "I will not post messages I later regret" :) .