I deduce you want to remove a logical-log, logs exhibit with command onstat -l, right?
If yes... you can remove with command "onparams -d -l <log_number>"
But with this command the log will marked with Delete flag, to complete the process you must execute a Physical Backup level 0 .
Now , if the problem is your logical logs are all without backup (flag B on onstat -l) , and you want to delete them... I think this is not possible...
You need execute a logical backup. If you do not have space on disk or tape to do this, alter you LTAPEDEV to /dev/null and execute ontape -a, or just link the file especified on the LTAPEDEV to /dev/null .
Exemple:
If your LTAPEDEV = /informix/archive/log/log.backup, remove this file and make the link: "ls -s /dev/null /informix/archive/log/log.backup"
execute the ontape -a