As far as I’m aware, Disabling the Shields, won’t disable the avastUI.exe (in the same way it doesn’t remove avastSvc.exe) as the OP is asking.
That is your way back in to enable the shields.
@ AussieEevee
The avast self-defence module would stop avastUI.exe from being removed. You would have to disable that first avastUI > Settings > Troubleshooting > uncheck the ‘Enable Avast self-defence module’ option and acknowledge the on-screen confirmation. Then you should be able to disable/kill its running task.