You cannot deny anything to a sysadmin.
You could try a DDL trigger on CREATE DATABASE, but the problem with those is they are AFTER triggers and I'm not sure if they work for RESTORE database as well as CREATE. I suspect not because it can't be rolled back.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability