Bell with slash means 'notifications off' or 'do not disturb' — muting a thread, silencing a group chat, or telling someone 'I don't want to hear about it'. Visually it's the bell emoji with a strike-through, and it's frequently mixed up with 🔇 (muted speaker) even though one is for alerts and the other for audio.
Keywords: bell, forbidden, mute, no, not, prohibited, quiet, silent, slash, sound