How to delete redundant Lightmass samples?

I have a level that shows lightmass samples, with out containing a LightmassImportanceVolume. I started Lightbuilding in that level until I decide to seperate objects into different Level, an use this Level as a container.
If I do Show-Visuailze-Show Volume Lighting Samples, I got this:

So, how do I get rid of the samples? Are they still taken into account?
Any Ideas?

Thank you

When you don’t have an importance volume Unreal will try to make one for you that fits all actors. If you want to limit it to a smaller area simply add one yourself.