What about hiring them as a mercenary to attack them for a certain amount of credits every turn. With the cost changing based on the admin/tech/civ level difference between the attacking/defending regimes

<t> I use the MNGT ASSET screen with preview activated all the time. It's really helpful when you need to know if you have enough stuff on hand for your construction projects.<br/>
Red means you lack it completely, Yellow means you fulfill some of the requests, and white means fully provided ...