I think a vague "boycott" will have little or no effect. However, in that vein, targeted or specific or short-duration boycotts would get noticed. Like the way Canadians are boycotting Florida and US booze.
Short-duration boycotts can operate like the massive demonstrations we've seen (which need to be and will get bigger as the economy bites). If 20 to 40 million people don't spend money on a designated Friday and Saturday, it will be noticed.
It's easier to get people to not do something (not spend) than to do something (go out for a few hours to protest), but the latter has been accomplished. It is harder, though, to organize a spending strike (publicity). Boycotts and street protests can both be accomplished; they are not at odds with each other.
Longer duration boycotts are of course even more effective, but much harder to organize. For those, specificity and targeting helps a lot.
But to your point: Save cash, buy fewer assets, sell a few more assets, conserve cash in case we head into a recession. This is a partner to your call for a "tRump boycott". In recessions and depressions cash is king. But it looks like we'll see or are seeing stagflation instead of recession, at least until the tariff issue stabilizes, which might not happen before Nov 2026. Or it might. With the inflation component of stagflation, cash depreciates in value and assets increase in value. But we might not see much inflation and it might not be stickier than 3%. We'll have to see how the Fed reacts through the end of the year and beyond. We'll have see if stagflation transitions to inflation or recession.