It's a common practice done on loud speakers and subwoofers. Some speakers even come with foam plugs for the ports. My previous pair was B&W 804N and they suffered from the similar mid bass hump in my room. When I tried the plugs it helped quite a bit. However 804N's are front ported, and plugs sticking out the port is an aesthetic disaster. I've replaced them with my current JM Labs, and the bass response is much more controlled now. No need to plug the ports any more.
You can first try with loose clothes and stuff the port covering the entire diameter. If that works, then you might want to custom cut foams for a better fit and looks.