If you use a script trigger on your drop down box, it can perform a script like this:
If [Yourtable::DropDownBoxField = "Closed"]
This is just temporary. Any find or Show All Records action will bring it back into the found set. You may want to use Manage | Security to set up record level access to better keep "closed" records hidden. See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a detailed description of how to set this up.
You can use auto-enter options on your field to auto enter "Open" into this field each time a new record is created. The data box in field options is all you need to use for this.
Thanks a lot, that works great