Remove 1:N relation between entities?

Hi,

seems I am missing something, please help me out.

Assume I create a custom entity with an 1:N relation to opportunities. If I create a list I can from that list then add a new or existing opportunity to the new entity. But there seems to be no way to remove a an opportunity from the relation again.

How can this be achieved?

Thanks in advance

Using CRM 2011 Online