"Toasts" are implemented in an app's java programming, so that needs to be modified in some way, assuming the app doesn't give you the option to configure toasts. The following assumes you're rooted.
The easy way: Use Xposed. There are many modules available to block all toasts, block toasts for a specific app, or even customize toasts, e.g. EnhancedToast and UnToaster Xposed. Xposed supports Android 2.2(Froyo) and up, with increasing support for 5.0+(Lollipop). If your Android is bootloader locked, aka no TWRP/CWM recovery, you can see if your device can run FlashFire to install Xposed. Note: To use Xposed modules on an app, it needs to be deodexed, e.g. data apps, apps you download from the Play Store are deodexed.
The hard way: You can usually decompile an app into smali code, edit the smali code to disable toasts, and then recompile the app. Without smali modification, this process is known as deodexing. If you're interested, check out this Lollipop compatible deodexing guide for decompiling an app's java code, and this smali code editing guide to learn how to edit smali code.