How can I force a CPU consumption limit? Say per user.... or system-wide.... both if possible?

You can limit the cpu time a process or user can use and the process priorities but you cannot limit a user to only use, say 50% of the available cpu cycles.

An easy way to set user limits is through pam_limits.so. The drawback is, that these limits are enforced on a per session basis, not system wide. So if you limit the cpu time to n minutes for a particular user this limit will be enforced on the current login session. If the user logs in m times he has m*n minutes cpu time available. Of course, you can also limit the number of simultanous logins through pam_limits. See /etc/security/limits.conf and man limits.conf.

If you need something more sophisticated you might need to turn on process accounting and write a script to automatically take action once certain limits are reached.

mmmmmmmm.......... well.... I think it would be OK if I set the nice level to a reasonable value... say......... 19 . Could that be set by root so that users can't set it BELOW the value set by root?

Yes.

man renice and see my last post.