ok sure to remove the rpmforge repo:
yum remove rpmforge-release
I think the fedora-release-15 package should be installed already. If not, you can download it from here
and then install it via "yum install fedora-release-15*.rpm"
It could be that it will remove some dependency package, when you remove rpmforge-release. Afterward you have to reinstall these packages via "yum install PACKAGE1 PACKAGE2...", if you need these packages.