Do you know about Edit - Comment Add and Comment Remove, which can also be assigned to hotkeys. This will add or remove a line comment on each selected line. Ok, it is not a block comment, but the result is the same.
To temporary remove a C code section I normally use
TRASH is not defined and so the code is ignored during compilation. I like this more, because the removed section is still highlighted normally, which is a big advantage, if I have to look at it again. Second advantage is, that I can simply search for this "comments" before creating a release version, to check, if I can delete it now or I have forgotten something to change back for the release version.
Edited on 2010-03-02: Starting with UltraEdit v16.00 there are the commands Edit - Comment Selection and Edit - Uncomment Selection to insert/remove block comment on/off strings on the current selection.