Does anyone know how to delete messages out of notifications?

I have gotten some messages from different people over the psn that are asking how old i am, what gender, you know the really strange people, i have left the groups and blocked them as you dobl and in my notifications that is all it says... you have left one of your groups and i assumed that was the end of it. However one of them still shows one message and who it was from but everytime i click on it to try and get rid of it, it says cannot show message, you have already left the message group. Is there anything i can do to get rid of it? Thanks