How do I block my computer from restarting for updates?

If it happens because of a Windows update, then try setting Active Hours. (Settings, search for active hours). You should have a period of up to 18 hours available during which restarts due to updates should not occur.

Rarely when i get to my desk in the morning my computer has restarted.

- could I check what you mean... do you mean this occasionally happens?

There was an update this week requiring a restart.

IF you check your Update History and the News thread here for each update to see if a restart was required, you can correlate the two.

If you are experiencing random restarts more than once a month or so, and not correlated with any update, that's occurring for some other reason.

In that case, check your Reliability History (Cortana search for Reli).