Those are usually just welded to the frame without any holes. I would hack saw them close, and using a disc grinder, grind even closer, and finish up with a file so you don't grind into the frame rails (don't ask me how I know this
) If you DO nick the frame rails, you can fill in the nick with some bondo and sand smooth.
But like mentioned, be sure you don't need them for mounting exhaust or brake hardware before you chop them off.