make "implements requirement" link mandatory while closing a Story
Accepted answer
As far as I know you would have to write your own EWM Work Item Advisor/precondition. This is a Java based Server Extension that would check the work item in the save operation and check that there is at least one Reference to that link.
You can find some sample advisors here: https://rsjazz.wordpress.com/?s=advisor&submit=Search they all reference to the Getting Started and the EWM/RTC Extensions workshop for how to.
Please note, the link we are talking about is an OSLC Link. As far as I know there is no mechanism available to prevent, that the link is deleted e.g. from the other end. EWM will not be called when the link is deleted out side of a work item save operation.