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Once an element or diagram is associated to another element, there's no way to delete the association unless the associated element or diagram or the element itself is deleted.
actually there's a reference node created under ther element node when there's an association. This reference node can be deleted to delete the association. However, the reference node is not created in the Beta 3 build. In the recent daily builds, this node is created sometimes and it also lost after some time if created. The problem has been created in issue 97025. So, if issue 97025 is fixed, there's an informal way to delete the association. There still should be a standard way to delete the association thru element context menu option.
this issue may be a duplicate of issue 87512, except deletion of element-element association