I wouldn't dump the oil out. That is only gonna cause bigger problems, first obviously is it is YOUR land that you just dumped a couple of quarts of oil on. Secondly, unless you leave them a note saying what you did, they will crank up and take off and once their bikes lock up they will be FURIOUS and even though they are in the wrong to begin with, they can seek damages against you. The smarter thing to do is simply let the air out of their tires, at least two of them. I doubt seriously they have anyway of re-inflating the tires out in the boonies. And I mean that, actually taking the valvestem cover off and pushing in the stem so that the air runs out. DON'T run your knife through the tires, that only causes bigger problems. A simply deflated tire is not damage and will not be of issue in court, but it is equally immobilizing as well as anything else.

I agree tho, in an IDEAL world we could just shoot them dead and feed the yotes, but that would create MUCH more problems than it would solve!
RA