in-Portfile variant removal (not default)?

[Michael Dickens]

For qt4-4.6.3, if I configure as +framework and +debug, 'configure' will
internally change the latter into +combined (meaning: debug and
release); in 4.7.0b2 with these variants, 'configure' prints an error
and exits.  I'm inserting some "ui_msg"s into the qt4-* Portfiles to
explain what's going on -- warning the user and/or providing info as to
how to use features of the install (e.g., getting access to the
FOO_debug libraries when executables are linked to FOO framework).  So,
what I'd like to do is change variants inside the Portfile from +debug
to +combined; I'm sure it's possible, I just don't know how & quick
search of Portfiles and the MacPorts Guide have come up empty (and, it's
possible I was too quick in looking).  I've tested the addition of
+combined and that works (via "default_variants" in side "if"
statements).  But how do I remove the user-specified +debug variant? 
Thanks! - MLD