Even if you mute a channel or an entire server, @everyone mentions will still bring up notifications. If a server is being particularly spammy with these all-powerful attention-catchers, you can suppress them to regain a little bit of sanity and peace of mind. Step-by-step:
1. Click the server name to bring down the server pull-down menu:
2. Click "Notification Settings", and check out the super spiffy channel notification settings list. When you're done with that, click the "Suppress @everyone and @here" option:
That simple. Remember, the "Suppress @everyone and @here" option functions on a per-server basis, so you can be selective about who you want to hear from depending on which server you're on.