Diagnosis
You didn't ruin it. This is uneven seasoning. Too much oil was applied at once, creating a thick, sticky polymer layer.
How to Fix It
Heat the griddle on high for 15 minutes to burn off the sticky residue. Let it cool slightly. Scrub the entire surface aggressively with a grill brick and water. Rinse and dry. Re-season properly: apply a thin layer of high-smoke-point oil (flaxseed, canola) with a paper towel. Heat to smoke point for 15 minutes. Let cool. Repeat 2-3 times.
If That Didn't Work
If patches are truly burnt-on carbon, use a palm sander with 80-grit sandpaper to remove the top layer. You're taking it down to bare steel. You must re-season immediately after to prevent rust.
Heads Up
The key word is thin. Wipe on oil, then use a clean towel to wipe it off like you made a mistake. The residual film is what you want to polymerize.