Stop leak blocking heater core?

[mcsprint71]

Had a slow leak in bottom of radiator and to get a bit of extra life out of radiator was advised to use a stop leak type product. A few days later the heater stopped blowing hot air and from the reading this list, I'm guessing the heater core is blocked from this stuff. What should I do? Will there be any long term damage if I don't unblock straight away and how do I it?