This often happens with unmanned strips. We have dogwalkers, horse riders etc.
The worst is kite flyers who just lay all their lines out across the runway to untangle them. Luckily, the last time it happened I was on the ground and was able to go and grab all the lines etc and bundle them up so an aircraft could land. The kite flyers got quite disgruntled that I'd undone their untangling. They didn't even seem to realise that they had no right to be there.
You do have to be careful about buzzing them as someone was prosecuted for doing this at Nayland some time ago.
ZA