From Ofcom site -
What can I do to stop receiving spam texts?
If you receive a text message from a sender you are familiar with, or from a shortcode (a shortcode is usually 5 digits long but can be up to 8), reply ‘STOP' to the telephone number or short code shown in the text message.
This will inform the sender that you no longer wish to receive their text messages.
However, if the text message is from an unknown sender, or from a sender you are not familiar with, we recommend you don't reply.
Responding to the text will confirm that your number is active and might actually result in you receiving more messages, or even voice calls.
Instead, you may report the text to your network operator.
To report a spam text forward the text to 7726.
You may get an automated response thanking you for the report and giving you further instructions if needed. You will not be charged for sending texts to 7726.
An easy way to remember ‘7726' is that they are the numbers on your telephone keypad that spell out the word ‘SPAM'.