There was a previous thread on this topic but the TS was using a TPlink router.

"how to block ppstream using router"

Maybe you could go thru the suggested ports and see if it works.

Happy New Year 2013.

What shelby_yong mentioned is valid as there is always a "way"

Issue the staff a company internet usage policy with HR help and mentioned disciplinary action would be taken against staff abusing internet usage for NON business usage.