1. Do not use Disconfirm
: to deny or refute the validity of
Up until reading this question, I didn't even know that disconfirm was a word, and I have a fairly large vocabulary. It's simply not something I've ever encountered or used before.
The word used far more often, as mentioned in the definition, is deny.
Google Ngram Viewer indicates that disconfirm is almost nonexistent in print, refute is comparatively uncommon, and deny is quite common:
So purely on the basis of wanting a word that is "quickly, easily and unambiguously understood," I suggest not using disconfirm.
In particular, in the same sense as disconfirm, deny is the common counterpoint to confirm:
"I can neither confirm nor deny that."
However, while it makes sense to click on a Confirm button, it makes no sense (with respect to the context in the question) to click on a Deny button—and much less sense to click on a Refute or a Disconfirm button.
2. Confirm pairs with Reject at the time of a decision
The most likely button label to pair with Confirm is Reject:
1 a : to refuse to accept, consider, submit to, take for some purpose, or use
// rejected the suggestion
// reject a manuscript
Although, in point of fact, Confirm itself might not be the best word.
Depending on the context of when these buttons appear, a more ideal pairing might be Accept and Reject
If there is only a single button (Confirm), then there is no reason to consider its opposite.
3. The warning itself should just use cancelled, aborted, or stopped.
Once the process has started, you would not then mention rejecting it after the fact. That would be strange. Instead, you would use one of the words already used in the question itself—or a third one:
- Once the Confirm button is pressed, the activity cannot be cancelled.
- Once the Confirm button is pressed, the activity cannot be aborted.
- Once the Confirm button is pressed, the activity is started and it cannot be stopped.
Note the pairing of started and stopped in the final version, which makes stopped the most natural choice.
In the first two versions, where started is not used, either of the other words is possible.
Turning to Google Ngram Viewer again, however, I compared it cannot be (cancelled / aborted / stopped). It showed that stopped is far more common than the other two:
In conclusion, if you want to use the most common and understandable phrasing, make the warning message the following:
- Once the Confirm button is pressed, the activity is started it cannot be stopped.