I think piano wire at neck height and a single warning sign.
Seriously we've been repairing the Gully ar Gisburn, there were explanation notices, 6ft posts with tape right across the entrance, 2 seperate barriers blocking the trail 15m and 30m in. None of it made any difference, we even had one cock ride down on us while we were repairing the trail. He actually rode past one group of us, over a blind crest and nearly landed on someone else. When told to turn around and walk back up he got very grumpy, not as grumpy as when he was the told in no uncertain terms to **** off.
Even the new section we're working on which isn't connected to existing trail at either end is being ridden! I've come to the conclusion that there are a lot of users that really appreciate and respect what the trail builders do and a small minority of complete ****s who actively seek to wind us up (like the c**ck on our facebook page who had a go at me when I pointed out the unfinished section wasn't yet open and was potentially dangerous).
We've tried felling trees, digging out but not surfacing as well as barriers, none of it works. People riding the dug out sections prior to us surfacing them really cause problems, we end up having to dig out the slop they've created making the trail even deeper.