Set Assigned To = blank/empty in customized module

Hi

I have a customized module where in some cases I want to remove the "assigned to", how do I do that?
I want an empty value in the assigned to field. In this case, if a certain value in a dropdown list changes value.

I have tried via BMP but it is impossible to set assigned to blank, it requires me to assign the post to someone.

Thanks Jorgen