How to limit the internet speed of others?

[Forasking]

The internet is used in a office. Some one is using it for watching movies during work time. And you know, it it not easy to say that you are not allowed to do so.

So I'm asking how to limit the internet speed in order to make her can't watch any videos?

Or some way to only block the port of videos and downloadings. Browsing the website is ok. 

I know so less about the modem setup. So if you can teach me step by step, I really apperatiate it!

Btw, I use WiFi in the office.

***edited labels***

[Gdkitty]

Unfortunately, while some of the Rogers modems have some limited time of day usage restrictions, there is not much else in the way of restrictions.
Even your most basic 3rd party routers doesn't have much more than that
You usually need to get a slightly higher end 3rd party router.

Many of the newer Asus routers have an interface like the following
http://event.asus.com/2012/nw/dummy_ui/en/Advanced_QOSUserSpec_Content.html
Which allows to set restrictions on certain ports or packet types.