Learn to read the room. This woman has text and repeatedly knocked on the door. She is not going to take a hint, especially when that hint makes no fucking sense.
They recently talked about it. The neighbour still thinks the OP is fine with it. The neighbour knows that the OP is totally aware she uses it. Hasn't given any indication that she is unhappy.
She texts to day the wifi isnt working. She gets a reply saying "mine is fine" so she will text back "its not working down here, can you check?" Or "its not working down here, have you changed/upgraded?".
Then the OP needs to send the message she should have sent in the first place.
Your little idea doesnt help af all. It doesnt avoid anything. It will invite more questions and make her look stupid when she actually explains the situation.
If they had never had those recent chats about the wifi usage then it would work. The OP could play dumb and say she didnt realise the neighbour still used it, but that wont work now because that is not the situation she is in. They have talked about it.
Everyone is telling you that you're wrong. It will not work.
The OP doesnt want to have the conversation but this is the situation she has got herself into. She can ignore it and hope it goes away or just bloody tell the woman, but a totally irrelevant phrase wont help her.