I think Karate Man is correct, and that anyone suffering police harassment should go immediately to the US Embassy and file a report and complain, and follow up. Things are too quiet there.
Then again, the Embassy might counter with their own warnings for the DR posted on the US State Department's website: Quote ": SAFETY AND SECURITY: American citizens should be aware that foreign tourists are often considered attractive targets for criminal activity, and should maintain a low profile to avoid becoming victims of violence or crime. In dealing with local police, U.S. citizens should be aware that the standard of professionalism might vary. Police attempts to solicit bribes have been reported, as have incidents of police using excessive force."
Regarding using your lightning fast reflexes against a Dominican cop, no matter what he's trying to do, good luck with that. You'll end up with a toe tag in the morgue faster than you can scream "Hai Yaaah", and more bullet holes in your dead body than Trujillo's last sedan. So much for respect.
In my humble experiences, humor usually works a lot better here than in-their-face reactions, and so does just saying "no".