At the moment there is no way to make a link control mandatory eg. force a link to be created.
It would be good if we could have a 'Mandatory' option when creating the control and maybe even a 'Must have at least x links' or 'Maximum of x links' type control
A good example of this would be linking contact to tickets. In most situations you would always want a contact linked to an incident however there is no way to enforce this.