Personally, I think the best, most subtle & non-political way to “disprove” or dent Keynesianism, specifically for the economically ignorant, is to point to ye olde principle of scarcity & the obvious occurrence of technology, science, & information constantly refining in consistent leaps every 6 months (to give a nice, general chuck of time for perspective, even though it can be easily be every 6 weeks on smaller scales), which means very easily, we all could be wrong on many things, given a certain amount of time to pass (such as the case of philosophy once attempting to explain what physics does in describing the physical world, etc.).
Keyenesianism is no different, & anyone who says otherwise, is either an idiot/ignorant, or has something to lose over it being disproven (as it has time & time again), such as their ‘kaumpy’ place in a newspaper site, or sales from selling over-priced books @ Wal-Mart, or embedded with DRM @ amazon.
The fact that at some point, someone is probably going to attempt to create an information-based or energy-backed currency (i.e. new things), also puts economic knowledge & information at risk, specifically since one cannot precisely design an economical ideology that fits with future circumstances & phenomena, which is the entire point economic knowledge is continually tested, developed, & argued for & against.
Keyenesianism would only win in a vacuum, & even then, it’s proponents would still have various inflated senses of worth over other “lesser” Keynesians who aren’t “Keyenes” enough!
Another thing to remind people is that history is not the past, despite the hammering done into anyone’s head in public schools. History is now, & if we aren’t careful, we’re going to miss it all before our eyes (as we are on the verge of missing the actual reasons for the incoming Blowback Period, which sounds less alarming than “Collapse” but just as correct in the effects of the 20th century’s “voluntary embrace” of Keyesnianism, Socialism, etc.).
In the long, Keyenesianism is dead, & quite ironically, it may take another 70 years for everyone on the globe to suffer under it “equally” for this to occur (hopefully, not). It’s not a surprise I’m typing this instead of sleeping, when I ponder such pessimism, but who knows, with the rate of technological progress, maybe Keyenesianis will eventually see no use for their “worse is better” economic ideology.