Bing Webmaster Tools Won't Let You Block Important URLs
If your URL is incredibly essential to Bing, it will tell you that you are out of luck - get another way of blocking the URL.
@Dingman posted a screen shot of the error he received after trying to block a URL on his site. It reads:
It seems you are trying to block URL(s) that we see as important. We do not support these blocks through the Bing Webmaster Tools, and we recommend you add a noindex meta-tag to your URL(s).
Here is a picture: