I'm going crazy trying to sort WhatsApp notifications on Lollipop - help, please?
The popup bar at the top is called Heads up notification. 

There are multiple ways you can disable them. For example there are ways to do it with root and without root. For more information how to disable them, look at this question: Android 5.x Disable Heads Up.
In the accepted answer of that question there is a link to an app that can disable them for you without root. If you purchase the pro version for that app you can also restore the notification ticker like in Android Kitkat and below and even select to only disable heads up for certain apps like Whatsapp. For more information about the pro version look in the apps description.

Hope my answer helps!