You have to work with the land owner and carry out his wishes. You can't put up posted signs without his permission, and I wouldn't confront any trespassers without his guidance/permission. Any contact with trespassers should be kept cordial and polite - informing them that they have accidently strayed onto private land... unless the land owner tells you differently. You don't want to provoke an incident on your own accord -- any retaliation won't be against you, it will most likely be against the land, and thus the landowner.

So, step one, find out what the landowner wants you to do, and follow his rules. Offer to post the land for him if he wants, and only call law enforcement if he wants you to.