I know following code will remove all package from specific repo.
yum remove $(yum list installed | grep rpmforge | awk '{ print $1 }')
And following code will remove a package without dependencies.
rpm -e --nodeps "php-sqlite2-5.1.6-200705230937"
But i don't know how to use together.
Print list of all repositories to get repo id (first column):
$ dnf repolist
Now remove all packages of selected repo:
# dnf repository-packages <repo-id> remove
See repository-packages section of dnf
manual page for details regarding manipulation with all packages in specific repository.