Type "kill -INT `cat /var/run/slattach.modem.pid`" (as root) to kill slattach. Then go back to kermit ("fg" if you suspended it) and exit from it ("q").
The slattach man page says you have to use "ifconfig sl0 down" to mark the interface down, but this does not seem to make any difference for me. ("ifconfig sl0" reports the same thing.)
Some times, your modem might refuse to drop the carrier (mine often does). In that case, simply start kermit and quit it again. It usually goes out on the second try.