Make sure your "Intersection Priority" (within your "Building Materials") is set to a higher number for the material you want to clean up. See attached
Robert J. Garand
ArchiCAD USA 27-Build 5001 USA FULL
Windows 10 Prof (64 bit) - Intel i9-10920X CPU 3.50 GHz - 128 GB RAM - NVIDIA Quadro RTX 5000