If your package cannot coexist with other packages (because of file conflicts, runtime incompatibility, etc.), list the other package names in the CONFLICTS variable. You can use shell globs like * and ? here. Packages names should be enumerated the same way they appear in /var/db/pkg. Please make sure that CONFLICTS does not match this port's package itself, or else forcing its installation with FORCE_PKG_REGISTER will no longer work.
Note: CONFLICTS automatically sets IGNORE, which is more fully documented in Section 12.15.
When removing one of several conflicting ports, it is advisable to retain the CONFLICTS entries in those other ports for a few months to cater for users who only update once in a while.