You'll either wait it out or completely change out your substrate. Waiting it out will take months...depending on the number of tabs you placed in the tank. It's been 3 months for me. You'll need to do larger weekly or bi-weekly water changes. I'm considering buy an additional HOB filter and rotating packs of Nitra-zorb and Purigen to help out.
I guess on the upside my frogbit, which struggled to grow in my tanks before this is now thriving. I'm tossing away handfuls a day.
Want to send me a handful or two to suck down my nitrates?