hello,

I want to know the procedure to close the thread, which is no longer visible to any one.

Thanks.

There is no way, to close/end specific thread from informix side.
Threads are handled/owned by sessions, so you can only kill the session(and ALL its threads) with the following command:
and in general...it is a very bad idea to kill only one thread (while remaining threads should be running) becasue threads typically share the same resources (memory, mutexes etc)...