Again, not to get too specific, but what is this saying to the person doing the cutting?
Trail closure? For a weekend? For a month? For motos? For bikes? All the trail? Part of the trail?
Still too nebulous in my mind. Think about the message (I'm in marketing, it's my job):
1. Identify the bonewipe here (i.e. the person doing the cutting)
2. Specify EXACTLY what is wrong (i.e. cutting cheater lines, not "new trail")
3. Explain what happens if they don't stop (i.e. WE lose access, forever.)
"Altering trails will get bikes permanently banned from this park. "
City park does get closed from time to time for Moto races or other things. Saying trail closure does not really specify what is at stake.
Just my 2 cents.