But when i remove it later: m_space.remove(floorPhysics), I realise that other objects are still colliding with it. Is this a bug, or am I doing something wrong? I do not encounter this problem when i use a Box instead of a StaticTriangleGroup.
This is a known bug; basically, the triangle entities that existed when the StaticTriangleGroup was removed will stay there.
However, as a hacky workaround, you can force it to remove triangles by changing the world matrix of the StaticTriangleGroup to something far away from the colliding objects. The worldMatrix property setter removes any current active triangles, and as long as its new position is clear, no triangles will exist. Then you can safely remove it from the space.
The StaticTriangleGroup, Terrain, and a variety of other components (most of the collision detection system) are undergoing a major overhaul in v0.13.0 which will eliminate this kind of problem completely.