It is easy to block off the vents from the inside. You can cut a piece of plywood (or foam board) to fill the hole, put a rubber tube type weatherstripping around the edge and put it in place. Hold it there with a couple of 2x4's that go across the hole from rafter to rafter. Nail or screw the 2x4"s in place through the rafters, not into the roof decking.

I would do this instead of removing the vents. Removing the vents means disturbing the metal sheathing that could lead to leaks. If it isn't leaking now, don't mess with it.

Having said all that, I'm not sure that there will be much benefit to blocking them off, but there isn't any benefit to keeping them as is either.

BTW, an alternative to cutting plywood to fit would just be a square of plywood that larger than the hole, put tubular weatherstripping on top in a circle just larger than the hole for the vent and attach with 2x4's as above.