You could login as that person and cancel.
If you go to the background tables and you can figure out which entry is which person, in the mdl_facetoface_signups_status table, there is a column called 'note' this is where the reason goes. Statuscode 10 is cancelation. This table does not include username or course identifiers, so you would have to find that session by backtracking through other tables. It is doable but time consuming and, as you are fiddling with the actual data and tables, dangerous.
1 find session id (usually available in the URL
)...go to mdl_facetoface_signups and find the signup id for that user (requires userid). Then go to _signups_status and search using the signup id--edit.
edit: I'm adding that I don't recommend this but it is possible. It is also a lot of work for very little payoff. I say this because...well, I've done similar stuff and it is rarely worth it.