I had the same problem with a Honda CR250 '93 (but I do not know your model, so check if they are alike) the solution was to put something solid (like a piece of pipe big enough for the bolt to fit in) between the bike and a wall (or floor) to make sure that there is no possibility of the bike to move when you hammer it.
Then -although it may sound brutal- a sharp blow with a hammer and a cilindrical punch is what will take it out.
Like stated before, disconnect the rear shock absorber.
Another thing to try...before trying to drive out the bolt, make sure that you have loosened all the mounting bolts for the engine. This will allow some extra play where the bolt goes through the back of the engine.
You can also lay the bike down on its side and run liquid wrench or something similar down the side of the bolt.
I've had older bikes where absolutely nothing at all works. The you have to remove the subframe and hut the bolt off at each side of the engine with a mini hacksaw.