After regularly getting preyed upon by the bums at the St. Paul station, folks who wait for you to buy your ticket, then approach you for money, I decided to take action. I called DART. The guy on the phone said they'd send someone out right away. I don't think the guy understood that this was a daily issue, but I guess I can call again.
By the way, headphones do not act as a bum talisman in Dallas.