I'm not sure Mesh offers the ability to delete a computer. They probably assume people would use the uninstall function and then remove the computer that way. However, I can complete understand there are times the computer dies before you can remove any items, which means their assumption is bad.
In a search of their help section, forums, and the Live Mesh interface at devices.live.com, I did not find any tool or help regarding this question.
I would recommend you try posting a question in the Live Mesh forum where a Live spokeperson (as opposed to a dead or even undead or possibly just not-live) may be able to provide either an answer or forward the request to the appropriate teams for resolution.