Standing still while they mock you says more than any comeback you could type.

Not responding gets mistaken for losing, when it usually reads to everyone watching as the opposite. A reply is proof the thing landed. Silence leaves nothing to push against, and people hunting a reaction lose interest quickly when none arrives. Composure is loud in its own way, and it does not require you to be unbothered, only to skip performing it.

Write the response if you need to, then do not send it. Put the phone face down and go do the next ordinary thing. Check in a week whether it still seems worth answering.