Is there a way to stop devices getting access to a iphone?

[Phil2008]

Someone has said you can block devices accessing stuff on your phone, but how to you do that?

Basically someone has access to everything on my sisters phone from their phone and she whats somehow to stop this without starting all over again with a new account, phone number etc?

 

[Phil2008]

This person had access to her phone and screenshot allsort off her phone, so now somehow the person has access to all stuff on her phone?