Can you damage your speakers by plugging bass port vent?

No, you will not hurt the speaker unless you jam something so far inside that you physically damage a driver or the wiring. Some ported speakers were sold with foam plugs for controlling bass response.

I have had more than one pair of speakers arrive from the factory with foam port plugs.  It will not hurt the speakers , it will be fine.   May not sound that great but will not hurt them.  

Wasn't there a "Macgyver" episode where he did that and improvised a bomb explosion?  I'd have my Swiss Army knife ready.

The sound might not be optimal but won’t damage your speakers

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.

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.

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.

All that bass energy, builds and builds and builds until suddenly, "KABOOM!!"

+1 dynamiclinearity. The "ported speakers have slow boomy bass" is a proverbial old wives tale.

@OP, with the K6 being ported through the bottom of the cabinet, I'm not sure that you will find an improvement overall - yes you can reduce the bass extension, but you will change the overall tuning of the loudspeaker and probably not in a good way. Personally, I've never found that plugging the ports in loudspeakers results in  net overall improvement.

You would be better spending some money on some bass trapping and some absorption for the first reflections. Acoustic treatment is cheap and you can still use it when you move somewhere more permanent.

The only way I could get my R11’s to play nice with my F12SE’s was to plug my R11’s two rear facing ports with the KEF supplied foam inserts. The plugs lessen the bass, but don’t seal.
Frequency response changes verified with room correction software. Then used mini DSP on sub to further flatten. Very good results. 

Try a Schiit Loki Mini and turn the bass down.