To make these lines disappear, the walls concerned must all be perfectly coplanar and of the same material. If it is not possible then you must add a additional wall over all or add a graphic modification by an graphic override or a simple white hatch (last resort)...
Christophe - FRANCEArchicad Designer and Teacher
Archicad 15 to 27 FRA FULL
OS 11.6 Big Sur - MacBook Pro 2017 - 16Go RAM
"Quality is never an accident ; it's always the result of an intelligent effort" John Ruskin