If the bolt head is sticking out, vise grip pliers, or grind two sides flat and use an adjustable wrench. If the head is sunk in and you use an allen head or something similar that will be a lot harder.
1985 Kawasaki ZN700 Shaft, 4cyl
1989 Yamaha Venture Royale
2007 Yamaha Royal Star Venture
Large vice grips. and tap it with a hammer to spin it if necessary. just in case. be sure to turn the bolt in the correct direction. you never know sometimes, but some guys get it wrong since the oil drain plug is under the motorcycle.