[jerf]

Having seen this now several times, I find myself wondering if people are aware that it should either be "dispense with once and for all the fiction that..." or "dispel the fiction once and for all that...". To "dispel with" is to use a thing to dispel, such as "Let us dispel the fiction that the sky is green with this spectrograph result." (although that is still so klunky I'd never say that).

[plorg]

The phrase is a meme that comes from an actual (repeated) quote. In the context of that original quote there have been plenty of people who have commented on its grammatical errancy, but in the context of the meme it perhaps would make less sense if you fixed it.