Ignore

To prevent seeing chat messages (including emotes) from another player. You can use the Ignore Player button in the Socials window to add a player character to your ignore list.

Generally used when harassed, or just annoyed by someone.

The ignore list can only store 50 names maximum, increased from 25 with Patch 3.3.0. This can be expanded via AddOns (like IgnoreMore), but new ignores may not have the same functionality.

Slash command usage
To ignore a player character:
 * /ignore name
 * /ignore name realm

To unignore a player character:
 * /unignore name
 * /unignore name realm