The geometry you're referring to is a nuisance indeed, but it can be removed. After importing the modeled element into ArchiCAD, you need to explode the element into a morph. Once morphed, you can take one of two paths to eliminate the unwanted geometry: 1.) Select the entire morph and then change the edges to hidden or rounded in the morph's properties; or 2.) select individual lines of geometry that you want to remove and hide just those lines.
This can be a bit of a tedious process, but I find it to still be quicker than modeling the element from scratch most of the time.
Hope that helps...