It's challenging because the optimal strategy for converting an individual is not necessarily what's best for society as a whole.

Risking Godwin's law, imagine the extreme case of Hitler. The best strategy to convert Hitler may well be Jewish people befriending him, rather than confronting him. But in the meantime, Hitler is radicalizing others and committing genocide. At some point, it makes more sense to just assassinate Hitler (or deplatform him, or depose him, etc.) rather than trying to save him.