I am accessing someones o2 account but can't tell o2 to stop it.

[furtler]

I'd prefer to close the account but its not mine so although I can change tariff and get texts and emails I don't know anything about the sim or phone.

[Cleoriff]

@furtler 

You wont be able to close the account if it's not in your name.

To cancel the contract on any phone, a text is sent to that phone.

It seems weird that you can access texts and emails if you know nothing about the phone or sim?

IF the contract is in your name then follow this guide to cancel

Guide: Cancelling Your Contract