Can you damage your speakers by plugging bass port vent?

My old B&W CM6S2 came from factory with a small selection of foam inserts for the ports to adjust bass. 

But if it sounds horrible in your concrete room, I doubt that plugging the ports will solve the issue. I would rather consider near field listening to take the bad room out of the equation. 

Thanks all....was concerned because there seem to be a lot of energy coming from that woofer....at normal listening levels you can feel the bass in that room.

I have seen those plugs with monitors but wasn't sure this is OK for larger units.

Plugging bass ports is a totally normal move if you're in a too small or bass compromised room go for it.

Every one is correct. There will be no harm done. But you will, of course, lose some of the lowest bass since that comes from the port. How much loss depends on how much you stuff. Experiment; try using various amounts of say Dacron or plastic foam, etc.

If you stuff it so there is no perceptible bass from the port you will also improve the Q of the bass; what bass you have will be tighter.

An old, fine example of a stuffed port is the Dynaco A25 loudspeaker, the best selling speaker of all time. It used a stuffed port and while the bass was not subterranean, the quality was excellent. SEAS of Norway built it for Dynaco and have designed a modern kit version, the A26, which Madisound sells as a kit. 

You can manage bass with partial plugs such as socks or a towell

fully plugging is a bit extreme.