Disabling user input in column after Sum X is reached

My thread title may not be helpful; it's the best description I can concoct. I have attached my .xlsx file.

I am creating a polling sheet in which a user is allotted 70 "points" to distribute as desired among one or more candidates. He will type these "points" into cells in column B.

I wish to limit input within that column to a total of 70, preventing the user from assigning more than that.

What do I need to do?