Alright, added your way into the list of bugs/features and specifically noted, that it can be used to close spellbooks.
You might be able to change the cancel button in one of the files at war3local.mpq
If you find the right file, then extract and modify it however you want, then you can put it in the map on the same path that it was in the mpq.
There's 2 possible reasons why this won't work:
1. There is no such file(I wouldn't want to believe that)
2. This file can not be overwritten.