you take the port out so you just have the hole use a small peice of wood but yet large enough to cover the hole by about 3/4 of an inch in each direction. then put silacone around the rim of the hole place the peice of wood over the hole and screw it down using about 6 screws on each hole over 4inches otherwise just use 4 screws
but depending on how you got the box if it is made for the subs you have in it, most likely when you seal the ports off the box will be to big for the subs! Subs require less space when in a sealed enclosure!
sealed and ported enclosure volumes are different for a given sub. you can't just port a sealed box, or seal a ported box and expect the sub to perform properly. an example is, for an average 12" sub, ideal sealed volume is about 1-1.5cu ft. for ported for the same sub, it's 2.0-2.5cu ft. having the wrong volume will make for poor performance.