How to delete a message from my inbox without opening it

[Loksel]

I got a random message from someone unknown. Since the user it was from appears to be banned, I figured it’d be better not to open it and moved it to the bin but that hasn’t cleared the little mail icon. Is there anything that can be done about this?

 

[PsychoJing]

You can block them!
That usually prevents me from getting messages from those people even if they tag me.
Maybe it will remove notification too?
Doesn't hurt to try anyway.