[@Inkarnate] Some people get really anxious and don't want to tell you to your face that yes, they're leaving your RP. Sometimes they don't want the backlash (Not that most of us would), they think it saves them face, etc.

Someone (name will be withheld out of mercy) wanted me to kill of their character in my DR rp, and I could tell by how uninterested in how his character died that he wasn't really feeling it. So I was like "Uh, you were going to have another character join, are they basically replacing this one?" And holy cow, he wrote a solid paragraph of apologies and requests for forgiveness! Some people are really paranoid around the GM for whatever reason. 

Anyway, I guess it depends on how tidy you want your discord to be, and how afraid you are of looking like a tyrant in front of your other discord buddies. If seeing their name doesn't bother you, I wouldn't do anything. If their presence is troublesome, or you don't feel it will make much of a fuss, I'd just remove them. They don't need to be in your group to DM you after all.