You are under no obligation to show someone who has decided to do you harm any sort of kindness. By their actions they have forfeited that.
Remove them from your presence as quickly as you can. Block! Block! Block! If an online platform does not, or will not, give you the tools to do this leave the platform.
If that proves too difficult for you, and I understand since we're all slaves to our ego gratification machine, escalate to the powers that be if you feel they posted something that can get them banned. Trolls hate having their voices removed more than anything else in their lives.
If you need to escalate further, use whatever means necessary. But be wary because the tools you use can be used against you.