You can make a 'cut' for a small 'dummy' ventilation cover. Hopefully, you didn't finish off the area yet. Assuming you didn't, first, go to the Home Center and buy a vent that's the size you need and will 'fit' the area in question. Remember, if you need to BLEED the line or get your hand in there, get a vent cover that's big enough to work inside of. Hopefully, you're using white paint on the finished drywall because the 'fake' vent cover will bend in real well. Then, box off the 'opening' in a similiar manner that 'jb' describes, except in this case, you only need to do it so you can screw in the vent covers into the wood frame you make. Then, you're done! (Well, almost ...)
If it happens that the piped-out area is 'leaking' cold air into the basement, you can stuff some insulation behind the cover, kinda 'wedging' it between the pipe(s) and the backside of the drywall/box. Be sure the Kraft side of the insulation is facing you. Now, if you look into the vent and can see the Kraft paper, you have a couple of choices to make the vent less 'noticible'. Turn the cover 180 degrees, or put some white paper IN FRONT OF the Kraft paper so you see 'white' when you look inside.