America’s biggest tech companies are zeroing in on Iran, scrubbing their online networks of fake accounts, videos and social-media posts by the rising cyber adversary aimed at spreading misinformation.

Alphabet Inc.’s Google on Thursday said it had terminated dozens of YouTube channels found to be pushing misinformation on behalf of Iran’s state broadcasting arm. The announcement marked the latest in a flurry this week from major online businesses detailing efforts to curtail foreign abuse on their networks, many involving...