Fencing is your only effective option. Encircle trees with heavy gage fence wire (11-14.5 gauge wire ; weave no greater than 1") at least 3 inches from the tree trunk. Use sturdy poles to prevent the beaver from pressing in the wire to get to the tree. Cut the wire so that it has spikes at the base that can be inserted into the ground (to prevent beavers from squeezing underneath) Otherwise, tack it down. Type III galvanizing will allow it to last for years.
Evidence is mixed regarding the sand and latex paint technique. Recent findings suggest it does not prevent beaver gnawing.
Of course, removing the beavers (consult state law) will also protect your trees. See Beaver trapping for details on trapping beaver.
