fowebe:
For the Isometric View, create a new Layer Combination where the Wall layer has a different
Intersection Group Number than the Beams and Columns layers. In the Help files see: Home > Configuration > Attributes > Layers > Use Layers to Prevent Intersections.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David