Simply painting at each end (in the grass) with white lines (as per a sports pitch) "X X" should deter most pilots - that is if the owner does not want to go to the trouble of ploughing the strip up.
I agree with most posts on this thread - we do NOT have the right to land at a private strip without permission, except of course in an emergency. If it's pre-planned then good airmanship dictates getting permission and finding out a local briefing prior to visiting (for example are then any areas to avoid over flight to save upsetting neighbours)