Originally Posted by BetterRed

Click unwanted format in Tag browser formats to get a Green +, select all books (Ctrl+A), then from Remove books dropdown select Remove files of a specific format from selected books - FWIW I gave that action a shortcut - Alt+Del.

Arrow down to the undesired format (AZW3, MOBI, whatever), hit space bar to select, and then hit Enter. There maybe another way to do it, but that's the way I do it.

BR