How to just block one person's LINE group posts without blocking the person entirely?

I'm in a LINE group. So is User B. User B posts boring things in the group. How can I block User B's boring posts, but not block User B's direct messages to me, in case one day they want to send me one? As a matter of fact, User B is an expert at saying "Good morning." And wouldn't you know it, they do so every time the morning sunshine hits them in the right spot. Cloudy days excluded. So I suppose I would have to rig up a *regexp* filter. That way I could still be aware if they were having a bad morning and needed help. Alas, this is not an email box that I could run *procmail* on, but the tightly sealed LINE app. So I suppose I would need a LINE to email robot...