The phrase "It's high time" is followed by a past simple tense, even though the meaning is present or future. It expresses that something should have already happened.
❌ "stop" – present tense (wrong)
❌ "had stopped" – past perfect (too far in time)
✅ "stopped" – simple past (correct for this structure)
❌ "have stopped" – present perfect (wrong tense here)
