Your basement return air should have a damper installed which should allow you to restrict the amount of air drawn in by it. If your house is older it may not have this however in all newer homes this damper is installed by the ductwork installers. If it is not it is not very hard to add one. The damper is slightly smaller than the opening of the return which will still allow it to draw air even when completely closed.
When i am normally balancing a new homes system i will normally close the basement return so that the system will draw more air from the finished parts of the house, and specifically the top floor or bedrooms where people complain about poor air circulation the most.
Now even if you were to completely seal the basement return, i don think it would cause any issues or excessive static pressure on the system. The furnace will pull more air from the other returns to overcome it. Also if you have a furnace with an ecm blower motor it will ramp up to try and overcome additional restriction caused by added static pressure (to some degree) which is normally caused by a dirty filter.
Also in summer time you will need to close all your basement supply registers for cooling. Your air conditioner is sized for your top floors, and the cold air will migrate to the basement anyways. I find a lot of people decide to just leave them closed year round when there basement is unfinished as they are rarely down there, and it saves them from having to remember to close them. This is always homeowner preference, but for the cooling season they must be closed.
So i would close the damper if you have one installed. If not i would either install one or have one installed by a hvac contractor. Just remember if you decide to finish the basement that you have proper return airs installed in it. And not to just close it up and forget about it.