Cold air return too cold...

Quest: I want to keep the cold air from my cold air return from freely flowing into my utility room downstairs.

Background: When I finished my basement a couple years ago, I had to add a cold air return. The 6" (or 8", can't remember) flexible insulted duct runs inward between the joists, then 10 feet in it dips under the joist between the HVAC ducting and runs back towards the outer wall. The run may be about 25' in all. Then on a shelf above the washer and dryer I have a bucket that the cold air returns terminates in. I heard this prevented the cold air from freely flowing in.

However, i want to finish the ceiling in the utility room and hook the cold air return up to a ceiling vent. i believe this will allow the cold air to freely flow. Can I put something to block the cold air from freely flowing but allow it to flow when the furnace is on? If I put a vent with louvers like a dryer vent to the outside, will that open when the furnace is on? How about just putting a cloth at the end? Will that stop the cold air from freely flowing?

Any suggestions?

Brian