depends how you installed it. if you went from rpm "rpm -e fluxbox" if it was from source you should still have the source files you compile, in which case go back to that directory and run "make uninstall" if you used slackaware packages, just use erm.. pkgtool? if you used source and it's not there well... either download the source again and hope for the best, or manually delete each file. fluxbox is a very small manager though, and can easily be left there.
incidentally, please try and search teh forums for answers before posting, this has been asked a number of times, thanks