Games : Can Steam be prevented from updating itself on Mac?

Is there a way to tell Steam on Mac to not update itself when it hasn't been manually launched?

I have now seen problems with this on two MacBooks. Without even trying to run Steam for anything, suddenly the system slows to a crawl, sometimes the entire screen even goes black, other programs I'm using stall with the wait timer icon, and then a progress dialog appears showing how far my unwanted Steam update is progressing.

Short of deleting Steam, is there a way to make it not update itself when not being used?