How to slow down my WiFi speed in Thomson Speedtouch

Page 43 of the user manual indicates that you can use address based filtering. If you want to block youtube this is likely the easiest way to do it. Keep in mind youtube is not the only major streaming video provider. You may want to consider Vimeo as well.

Deny access to a specific web site
Proceed as follows:
1 Browse to the Thomson Gateway GUI.
2 On the Toolbox menu, click Parental Control.
3 Make sure the Use Address-Based Filter check box is selected.
4 Type the URL of the Web site you want to block (for example “mail.provider.com”) in the Web Site box.
5 In the Action list, click Block.
6 Click Add.