There is very little to be accomplished at the store level, were grunt employees are probably completely oblivious to the politics of any protestation you might make, and would be unlikely to relay that message to the people who need to know if they were aware of it.
Better to just cease patronizing them, tell every conservative you know, and then write, call, email the corporate office to let them know they lost a lot of business.
I’m not saying do this at just one store, or a few stores, but company wide. A boycott won’t work on a national level. They’ll get enough business despite a boycott, with perhaps exceptions in specific areas. Having to throw out a lot of product nationwide makes a bigger impact.