If you find your stock being used without credit and notification, send a ticket to the Help Desk. Do not use the Intellectual Property (DMCA) option - for exact reasons unknown, here on deviantART it does not cover this situation.
When you fill out the ticket, avoid referring to it as a copyright violation. Regardless of how you might view such a violation, it apparently limits the staff's ability to deal with the deviation. Just clearly state that the user has violated your terms, how they have done so, and link to the violating work and your rules. This should be the easiest and fastest way to get the violation removed.
Update: :devreali