Flash movies are Lan bandwidth killer

Hello,
we have Windows 2003 Std and Citrix PS 4.0.
Our internet connection is a 1,5 MB Lan connection and round about 30 Igel thinclients. I look at the switch i can see 20% traffic during normal work.
If a user open a website with a flash played movie, the traffic jumps on 100% and all
workstation have slower connections through the internet as long as the website is still open.

Is there any chance to change that, perhaps within our Citrix Presentation 4.0 oder with a IE7 Policy ?
The problem is, that we sometimes need the information on sites, which provides information for us
only over this kind of publication, so blocking is difficult.

Best regards
insi01