From the RuneScape Wiki, the wiki for all things RuneScape

An example of the Ignore list interface.

The Ignore List is a part of the interface that players can use to block messages from up to 100 specifically-selected players, especially those that the player finds annoying or abusive. A player "ignoring" a specific person cannot see that person's text in public or private chat. However, unlike turning Private Chat "off", the player doing the ignoring does not appear offline to the person being ignored. If the person being "ignored" sends a private message to the player, a message stating that you are on the ignore list will return to you.

Contrary to popular belief, the ignore list does not work like the friends list in that the amount of people you are able to add to it stays the same when you become members.

If the ignore list is filled to a near-full capacity, glitches will begin to occur in which the phrases and/or words inside the list begin to stack on top of each other.

Previous versions of the Friends and Ignore List Icons.

Trivia

• Some players used the Ignore list to make personal notes; however, this is now obsolete due to the 8 July updates that brought the ability to use Notes.
• On the 8th of July, along with the Mobilising Armies update, the Ignore list icon has an update to its appearance.
• On the 8th of July a glitch was discovered; by clicking 'Add ignore' and adding your own username with a space or underscore before or after it, you were able to add yourself to your ignore list. As a result, any messages you typed were hidden and you were rendered unable to enter your own clan chat. This glitch has since been fixed.
• As of Oct 1st, you can no longer write down notes/ add non existing names to your ignore list. Any names that don't exist before this update on a players ignore list have a star next to the name.

See also

• Friends List