Define Keynesianism.
Keynesian isn’t just a pejorative term for all the economics you don’t like. You can’t refute something you haven’t defined.
If you mean the conept of sticky wages, that will be impossible to disprove theoretically. Sticky wages are an empirically based assumption and so would need to be countered with evidence.
If you mean fiscal stimulus as a way to relieve falling aggregate demand, that has already been discredited by almost the entire economics community, despite what certain public figures would like to have you believe. Most New Keynesians don’t believe in fiscal stimulus.
The only other aspect of Keynesianism I can think of that still gets mentioned is the liquidity trap, which relates to barriers on monetary policy. If anything, this idea should be championed here.