Hi @Jeroen52,
We've built Mattermost as a real-time messaging solution and WebSockets are core to the user experience as they allow for the user to receive messages, notifications and other events practically immediately. Without them the user experience is greatly deteriorated and, as such, we don't have any options to disable such alerts in the system.
If you like you can modify the Mattermost source code where these alerts are displayed and compile it yourself or place a [feature request].