"Please do not brush your teeth here."
While the particle は after で is not necessary, it's not wrong either. It just gives it a slightly different nuance. I would still think it should be accepted though.
ここで歯を磨かないでください。(Simple statement, don't brush your teeth here)
ここでは歯を磨かないでください。(Don't brush your teeth here (but brushing them elsewhere is fine)).
My first thought was also to type what you had put.