How can I further block our vents in the basement?

Since it's summer time, we've had the AC on a lot. We live in a 2-story house with a basement, and the top floor never really gets cooled because all the AC air is going into the basement through vents in the basement. I've closed all the basement vents, but as I'm sure you know a lot of air still leaks out. So our basement is like Antartica, but the top floor barely gets cooled. Is there anything I can do to the basement vents to block the air better? Like duct taping it or putting a block of something in there? Thanks