I haven't done this specifically, but you can block uploads to any destinations you desire, via the "IronPort Data Security" policies. You'd need to create a custom category that matches the yousendit upload servers.
You can find out what these server are by uploading through yousendit while tailing the logs:
To tail + grep the access logs for this entry, run the following from the CLI:
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1. Tail
2. Enter the number of the log you wish to tail: 1 (for accesslogs)
3. Enter the regular expression to grep: .*
4. Do you want this search to be case insensitive?: Y
5. Do you want to tail the logs? Y
6. Do you want to paginate the output?: N
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