Component revision problem

Hello,  we have packaging items that we use for our assembly.  We want to include these in our BOM, however, whenever I add the item to the BOM, it automatically defaults the most recent revision to the BOM and I am unable to update it.   

We have multiple revisions of our items and we continue to use the old revisions for a period of time until those items have been fully used up.   We want to leave the revision field blank, that way we can choose which revision at the time of issuing components, however, the form doesn't allow us to remove this revision, and when I issue components, it only allows me to issue onhand quantities of the revision specified in the BOM.