I've just impoverished one barista at Starbucks because today I went to Cafe Nero instead! And I'm no longer a regular at my local Italian joint, so I've just deprived a few more people of an income.

For some reason you think this is about sticking it to the man but it's not. The staff get a stable, living wage and the business is charging prices much more in line with reality. I'm also no longer personally liable for the income of whoever served me, because the rightful responsibility for that lies on the employer.

As an individual customer, not likely since line workers' pay does not directly depend on the number of customers served (at least until it's so low that the business goes under and they have to move to unemployment benefits instead). Tips depend on each customer though, so if you chose Starbucks over Cafe Nero, and Starbucks barristas don't get tips, but Cafe Nero's ones customarily do, then yes, your choice had influence on the income of Cafe Nero's barrista. Probably a small one, but it's there. If neither barrista gets tips (I've never seen anybody to tip a Starbucks barrista but in smaller shops it happens sometimes, though much less frequent than with waiters) then there's not much difference from a single individual action.