You can block a soul from their profile, from your conversation, or during a call; blocking mid-call also ends the call.
A block cuts both ways: they can no longer message you or join Quests with you, and the matcher will never pair the two of you. They are not told. To them you just become unavailable, the same neutral state used in many situations.
Your blocked list lives in Settings under Blocked souls. Unblocking is one tap: you can message each other again and your previous connection resumes. If someone behaved badly, consider also reporting them: a block protects you, a report protects everyone.