Fedora, Ubuntu and Slackware are all Linux - not UNIX. *BSD, Solaris, SunOS, HP-UX, ..., those are UNIX.
Slackware would provide the best general idea of what you're looking to do, in terms of similarity.
HOWEVER, I believe the command to remove a package is "pkg_remove [package name]
" (or something to that effect). Make the FreeBSD Handbook