Deletion not possible; Rule &1 is still used in activity &2

What causes this issue?

You want to delete the rules for account assignment transfer &V1&.
This, howver, is not possible because the rule is stilled assigned to
activity &V2&.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

In order to delete the rule, you must first delete the assignment to
the activity.

Error message extract from SAP system. Copyright SAP SE.