Oracle 10g – How to prevent one user from hogging all resources?

I am wondering if there is any way to limit how much resources one user can take on my Oracle system. I want to limit some queries and systems from being able to hog all system resources when they run non optimized queries.

There is a built-in utility called Resource Manager exactly for that purpose:
http://download.oracle.com/docs/cd/B19306_01/server.102/b14231/dbrm.htm#i1010776

If you’re looking for something simpler than the resource manager, try Profiles.

I haven’t had much success with the resource manager. In my experience with it I’ve found that it’s easier to give priority over resources to certain users than to limit users.