Are you talking about the chat heads that puts a little avatar icon on the edge of the screen, or a full banner type notification along the top edge that only displays for a few seconds. If it's the chat heads, that can be disabled in the settings. If it's the latter, heads up display, there's no setting I'm aware of within the app to disable it.
From what I understand of heads up notifications, they're part of the OS (though, I thought it didn't come out until Lollipop) and apps can be programmed to use it. I don't know of many people that like it and it gets in the way a lot in my use. I think I've read some inklings about rooted users being able to bypass it, but I don't know how. If no one else here can help, you may need to Google search for solutions.