Phone customer services, press the button for 'Thinking of leaving us' when you get to the human being - do not be polite, but be firm and tell them that if they insist on keeping sending the messages this constitutes harassment and is illegal. Tell them they have 24 hours to rectify the issue, or they will be in breach of contract and also be breaking the law, you want compensation for the issue and for the stress it has caused you, and if it is not resolved in 24 hours you will leave and seek another provider, and add that the phone conversation has been recorded. If they quibble over the recording, then ask if they have recorded the conversation, if they say no, then remind them that they had the chance as it in their terms and conditions.
Under the law you can accept 2 of the same messages, after that it becomes harassment even if it is information.
Link -
https://www.legislation.gov.uk/ukpga/1997/40/contents
And from Citizens Advice:-
'Harassment is unwanted behaviour which you find offensive or which makes you feel intimidated or humiliated. It can happen on its own or alongside other forms of discrimination.
Unwanted behaviour could be:
spoken or written words or abuse
offensive emails, tweets or comments on social networking sites
images and graffiti
physical gestures
facial expressions
jokes
You don’t need to have previously objected to something for it to be unwanted.