Every time I buy a new (used) 240, I seem to run into this problem when I do the first tune-up (never a problem subsequently because I use antiseize when I put my rotors on).

Anyway, here's my foolproof (so far) method, which cannot damage the distributor or rotor, but will separate them: you need a weight (anything heavy, but I use my wife's 5-lb exercise dumb bell) and a long thin but very strong rope (about 4 ft, and I use a small sailboat's halyard) -- don't use nylon because it stretches, and you don't want the "springy" effect.

First tie the rope into a large loop, and then take a small portion of that large loop and wrap a couple of turns (almost a figure eight) around the two "ears" (the contact end and the counterweight end) of the rotor. Likewise wrap a few turns of the rope at the opposite end of the large loop around the weight you're using. Be sure to have about 18" or more of slack between the rotor and the weight, and have an equal length of rope on the two segments of those slack portions (you started with a loop, right?).

Now, give a very quick and forceful upward stroke holding the weight. Assuming your rope is strong (so it won't snap), the momentum of the weight and shock of the suddenly taught rope lifting the rotor will free the stuck rotor!