Insurers are no longer permitted to refuse to enroll you in a Marketplace plan because you owe back premiums. Report this problem to the Marketplace and to your state insurance regulator and ask their help resolving the problem.
In the future, if you should need to disenroll from a plan mid-year, it is important to actively disenroll and not just stop premium payments. In HealthCare.gov states, you can log into your Marketplace account and select the option to terminate coverage. This will also create a written record that you took this action.