Tele Block is a members-only spell that requires a Magic level of 85 and can only be cast on another player in the Wilderness. If successful, it will prevent that player from using teleportation for 5 minutes. However, if the targeted player is using the prayer Protect from Magic the duration of the tele-block is about 2 minutes and 30 seconds. The victim will receive a message in their chatbox stating that they have been teleblocked. The teleblock effect itself is applied at the moment when the message appears, and can't be dodged.
If you are tele-blocked, teleportation spells on your spell list will be black, as though you do not have the required level. However, The teleblock is removed when the affected player leaves the wilderness. Note that while the player is affected by the spell, the tele-block cannot be recasted until the spell has timed out leaving the player the option to teleport away after the time is up.