How to lock or erase a stolen phone that is not ,and will not be connecting to wifi

my phone was stolen and the person that took it knows I can track and control my phone if it connects to the Internet. So I need to do something to prevent him from flashing my phone(making me unable to locate or connect with the phone ever again)... or even showing personal pics and messages to everyone, im not very knowledgeable or up to date with a lot of the newer technologies and capabilities available these days. So I'm really hoping there is a half-way understandable way for me to take control of my phone before it's too late.please help