smbd is a inetd service, so you are right: it really is stopped and restarted, according to the requests it gets.
To prevent this you can run it in daemon mode if you like, although I'm not sure Amiga samba supports this properly; to do this, first disable it in Miami/Genesis inetd pages. Then run it like this:
Run >NIL: smbd -D
If it works, you can set it to in a script thats automatically executed when you go online.
However, all that won't stop it eating your CPU time. I advise you to use some tool or scheduler to reduce the priority of the smbd task to -1.