Sorry, there is no known way to remove the glue stains from the Cape Cod Houses. The glue get's into the plastic itself and cannot be removed. Trust me on this.
For the sake of conservation/preservation and the need for these houses to look good on a layout, I suggest using water colors to help hide the stains. It does the trick and can be easily removed should it be sold to a serious collector.
Joe
www.plasticvilleusa.org