UnitedHealth Group Inc. has stopped marketing an insurance plan to Massachusetts senior citizens with disabilities after state officials received dozens of complaints that the company was using abusive and misleading sales tactics. Senior citizen advocates said some UnitedHealth sales representatives refused to leave people's homes without getting a signature on a policy, and added others misrepresented the plan by claiming it would pay for care that is actually not covered. In addition, the advocates said, some agents repeatedly called seniors, despite requests from family members that they stop.