Hi,
onmode -z is the best way to clear an informix process.
onmode -z <session_id>
This is similar to Unix kill -9 <pid> interrupt signalling. Onmode takes care about the trasactions in progress condition too, if so it will judge the stage of the transaction, and tries to commit, if possible. But in most of the cases it rolls back the transactions. Such cases it may take a longer period to terminate a session.