Unlocking a serial port that remains locked after reboot

I am using pppd to communicate with a modem coected to a serial port. Sometimes, due to power failure or watchdog resets, the coection to the modem is not closed over a reboot.

pppd is then unable to communicate with the serial device on /dev/ttyO4 because it's apparently locked by another process. The screen command won't open it either for the same reason.

How can I ensure that I clean up all serial locks on this port when I boot to avoid this lockout scenario?