This problem has been driving me nuts for years, in my largest models it was impossible to right click and delete materials from the SU Material Editor. A real pain when SU automatically creates solid colour materials for imported dwgs.
Digitalmarley's trick above works! I had to change my Windows 7 theme to Classic (Basic didn't work), and SU stills hangs for about 20 seconds (for each material!), but finally the right click context menu comes up and I can choose delete. Thank you Digitalmarley!
Trimble really need to sort out how SU handles materials, it's a terrible bottleneck.
/Jackson