Can you damage your speakers by plugging bass port vent?

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. 

Re panzrwagn - Closed box sound is dryer because it ends faster, has less overhang. It's more detailed because it stops more quickly. Speed of bass is really not the initial attack but how quickly it stops and closed boxes stop faster. It's just that the extra overhang of reflex boxes adds richness like amps with high second harmonics that people like despite being less accurate.

With modern programs there are good reflex loadings(albeit less good than good closed loading). If reflex bass is too boomy it's either incompetent design or often unfortunately achoice to make a speaker sound bassy because that sells in way too many cases.