I've had that problem before too. I'd ask politely if he (she) could remove the bags and place them in the overhead compartment. The airplane boarding logic is that if that person is sitting in the front rows, that passenger is probably a ff elite member and has the privilege of boarding first. But very often I see F/A warning passengers about their bags and taking the liberty of putting the carry under his or her seat, by YOUR feet. If that happens, you lose.