I was reading about this and on websense site they talk of multiple ways to block Tor but they also mention that Tor sites registered are around 1300 so at the firewall I don't believe that regex level will help. I am going to check with CSC module that does URL filtering over HTTP/HTTPS sites.
I believe that CSC can block based on a comment that we have a fix for CSC function that did not drop it in the past.
5540 by itself cannot block Tor based on the fact that it works over HTTPS and ASA has no inspection for HTTPS, if you have a CSC module on the ASA that could drop it but it seems that you should be reaching out to websense since they should be able to block HTTPs as the firewall can do HTTPS redirects to Websense filter.
Due to the nature of the application I would not think so!
Use a dedicated application filtering device for it like the CX option on the Firewall (Application aware).
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