How Do I Remove Some Packages From rhn_check List Before Updating

When I installed RHEL 6 last year I included the modules required to run an HTTP server.  Since then I have done manual updates to Apache to level 4.2.3, beyond where Red Hat is at the moment.  When I do a yum --check-update I get a number of hits on httpd at lower levels that where I am.  The result is that the httpd server stops working (wrong init.d/httpd, wrong config overwrites the current one, wrong Apache modules selected, etc).

I can't back httpd out because it is a pre-req to a number of other (unrelated, and in some cases unknown to me) programmes.

How can I block yum from updating httpd?  (and some other services in the same situation, like pgsql)

Any help would be appreciated!

John