you dont need to maintain a flow between pump and that port , it doesnt flow when the heater is off
if you want to be certain it doesnt flow just loop the hoses at the heater core end of the heater rail assembly
if you're determined to remove the heater rail assembly
easy way is remove the rail (1" wrench ) put a socket on the fitting with a 1/2" wrench ,when the fitting is out put a 1/2npt plug there , for the waterpump just shove a copper plumbing cap into the hose end that fed the heater rail and clamp it
but i have to say again.... if your heater diverter valve is functional there is no flow through the heater core and nothing needs to be done for summer