How to fix/prevent blocked cleat bolts
Try a pipe nipple extractor or similar stripped screw extractor (Example:  http://www.plumbingsupply.com/extractors.html).  It is designed to grip the inside sides of a pipe using the same turning direction that will unscrew it.  It shouldn't need much depth to work and it will try to drill itself in.  You may still have to drill some of the junk out, but go slow and select a bit that is smaller than the head opening.