Keeping in mind the fact that "they can't do that!" is maybe a little naive these days ...I'm pretty sure that neither Johnson nor any other Speaker actually has that power.
Powell v. McCormack (Wikipedia) determined in 1969 that only Congress as a whole could expel members, and then only
after them having been seated and sworn, and only for reasons having to do with Constitutionally specified qualifications:
I guess Congress could, as a body, look into whether the member "had been duly chosen by and through the laws of their state,", but even that only after he's been sworn in, and not solely in the judgment of the Speaker. If even Congress as a whole lacks the power to deny an elected member his seat for "election irregularities," I doubt Johnson can, by fiat, unilaterally do so. As I said, it's not beyond the pale for the GOP to try, but...we all know what sticklers for Constitutional originalism conservatives like Johnson are, right? Surely they would stick to principle and forbear that particular tactic.
(And, of course, the usual disclaimer that IANAL, and may be reading all that all wrong)