Question about yum update

Hi guys..

I am confused.. few weeks ago I installed a test server with 5.8 Beta... then I ran yum update and it updated the system  to 5.9...

Now we are running a server with 5.8 and it SHOULD NOT be update to 5.9 because that is requeriment for the application it running on the server..

I thought yum update only update the packages for the installed version and not upgrade the OS...

is there any way to forbid the use of yum update?

Thanks