how do i logut from my mymtnapp in my stolen phone?

Hi Jerrald. Sorry that your phone was stolen, the best thing is for you to contact MTN support for them to block the SIM for you.
You can contact MTN support via any of these channels:
1. MyMTN App
2. Facebook: @MTNLoaded
3. Instagram: @MTNNG
4. Live Chat: http://bit.ly/2OC5bHz
5. Email: customercareng@mtn.com
6 .WhatsApp: 09033000001