EyeQ Constant SMS for Appointment

I have received multiple sms's from EyeQ to go get an eye test performed, normally within the first couple of days of the month. Every single sms has an option to reply No to opt out. Not to decline an appointment, but to opt out. I have replied with every combination of No that I thought might be required - "no", "No", "NO", No, NO, no - and I am still getting sms's.

Awful IT service. Can't even block the number as it changes each time.