It's very simple. Click on the user's nickname and then click on the icon that looks like a block/stop signal. Then confirm.
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I already tried that before.
There are 2 small icons, neither of which has a tooltip or look "like a block/stop signal"
1 look like a ! in a circle, the other look like an 'end limitations' road sign.
Playing on smartphone, if it is different on others plaftforms.
But I drew my conclusion from this reluctant support....