FreeBSD port manual
Use pkg_info to list all installed packages:
pkg_info pkg_info | less
To delete or uninstall packages / apps use pkg_delete command:
pkg_delete pkgname pkg_delete screen-4.0.3_6 pkg_delete 'screen*'
2nd one will delete package called screen-4.0.3_6 and 3rd command will delete a screen package using wild card.
Also, you can uninstall the same using ports system. For e.g. to delete screen port, enter:
cd /usr/ports/sysutils/screen make deinstall