The answer is: it is ambiguous between the two meanings, and one must use context to determine which meaning is intended. (This is, of course, accepting that uncancel is a real word. I am perfectly willing to say it is; in the past decade or so it has become a very real word in the computing realm.)
This ambiguity is because we have two un- prefixes in English. One attaches to adjectives and means "not" (e.g. unclear), and the other attaches to verbs and means the reverse of the action of the verb (e.g. untie). When you have an adjective formed from a verb by a suffix, then it may be impossible to tell how deeply the un- prefix is attached; that is:
So, if both forms make sense, as they do with un-cancelable and uncancel-able, then we have ambiguity.
Another good example is unlockable ("not able to be locked" or "able to be unlocked"). Yet another one is untied ("never been tied at all" or "tied at one time but then untied").
Technically speaking, the ad-hoc word for "not able to be uncancelled" (assuming you really want to use one word), would be ununcancelable. For the sake of readability, I would recommend writing it as un-uncancelable; that said, the best approach would probably be to use more words :)