Is there any way to stop Steam client updates?
You probably don't want to prevent it from updating, since that means it will eventually stop working. Which in turn could mean that games relying on auto updates might also stop working, at least in online modes. If you insist, here's some tips in the Steam Community. I haven't tried them personally and can't vouch for them.
It might make more sense block it from auto launching and starting annoying updates at computer boot-up:
- Top left corner: Steam -> Settings.
- Interface. "Uncheck Run Steam when my computer starts"