Can you damage your speakers by plugging bass port vent?

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.

No

A ported speaker will roll off at 24 dB/Octave below it’s tuning frequency. A sealed box rolls off at 12 DB/Octave, typically a half octave higher. An optimally tuned system that’s flat to 40Hz will be -24 dB at 20 Hz. The sealed box will be flat to 60 Hz, -12db at 30 Hz. Ported systems are harder ( but not impossible) to tune properly and so their reputation.for boomy ill-defined bass. Sealed boxes reputation is for tighter but ’slower’, less lively bass. It’s way more involved than just sealing the box. But if you understand the basics it makes what you hear so much more comprehensible.