Hey! I understand both your points but if an aircraft is in an active airspace and not responding after a few ‘On-gaurd’ messages, they will be reported. No need to give them instructions.
For situation n° 1: an aircraft being on final without prior communications with ATC will be on-guarded 2 or 3 times before getting reported if they are disturbing the flow, or disconnected if they are innofensive.
For situation n° 2: If an aircraft taxi without permission, they will be reported, there is already a violation corresponding to a Taxi without permission.
An on-guard message is already a lot of pressure, after receiving this twice or thrice, consider yourself violated already…