How can I solve the intersection between a wall and a slab?

There are two ways to solve this kind of intersection, using wall layer offsets and subtracting solids.

Wall layer offsets

Choose the right value for the layer bottom or top offset. This wall property can be defined by object or by style.

Subtracting solids

Use the _vaSubtractsolidsVisualARQ subtract solids command icon command to subtract the parts of the wall overlapping with the slab.
Note Learn more about the _vaSubtractsolids command