I have long experienced the inability to mute someone in my aiming circle, and I hear other people say it a lot too. Recently, I figured out a way to do it! And in case it hasn't been explicitly posted, here it is: On your turn, if you need to mute someone inside your aiming circle, first click on your weapon in order to open your weapon selection tab. Second, while your weapon selection tab is open, mute the player! Its that easy.
I think the solution, like Mario mentioned, would just be to have the option of removing the aiming circle.