I can't get over the gut feeling that it's a bad idea, but here goes...
First, make sure that some other user, preferably some DBA (yourself?) has SA privs.
Second, open SSMS and click on {View}{Object Explorer}. Expand {security} and then {logins}. Double click on SA and a new window will open. Click on {server roles}. Find the {sysadmin} role and deselect it. Click {OK}.
--Jeff Moden
Change is inevitable... Change for the better is not.