roles permissions about relationships

[veronicakatz]

Hi, everybody.
I'm working with roles, defining them and configuring their permissions.
But I'm having a problem concerning the relationships.
Let's say I have a "project" and a "case" related to it.
I configured a certain role not to be able to edit neither the project nor the case, but it can list and view them.
When a user click on the project, it shows the relationships, as well as it doesn't show the edit buttons (it's perfect! it's what I expected!) BUT it shows the delete button at the right. So, this user can delete the relationship between the project and the specific case!
How can I configure the role so as it only could view and list the projects and the cases?

5.1.0 CE + spanish language package 5.1.0

Lotsa thanks in advance!
Verónica