Use the REVOKE statement to remove permissions.
Example
REVOKE SELECT on SomeTable TO Public
Specific statement depends on what the objects are.
Directly editing the system tables, even on SQL 2000, was just asking for a corrupt database. I had to clean up often enough after people who just wanted to quickly edit the system tables and not care about the side effects (which can be a completely unusable database if you're not careful)
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