If you have access to an ANSYS Research license, such as in a University lab, then you can open your SpaceClaim document and do a File Save As STL on that computer. If you don't have access, then use a different piece of software to edit the STL file.
If you attach the .scdoc file (or put it in a .7z or .rar archive), I can take a look at why you can't cut.