If you turned on"Find my iPhone" BEFORE it was stolen, you can send a remote erase command that will take effect as soon as the iPhone connects to the Internet. The following is a C&P from the iPhone-6 user manual:
To use Find My iPhone features, Find My iPhone must be turned on before your iPhone is lost. iPhone must be able to connect to the Internet for you to locate and secure the device. iPhone sends its last location prior to the battery running out when Send Last Location in Settings is turned on.
Use Find My iPhone. Open the Find My iPhone app on an iOS device, or go to www.icloud.com/find on your computer. Sign in, then select your device.
Play Sound: Play a sound at full volume for two minutes, even if the ringer is set to silent.
Lost Mode: Immediately lock your missing iPhone with a passcode and send it a message displaying a contact number. iPhone tracks and reports its location, so you can see where it’s been when you check the Find My iPhone app. Lost Mode also suspends the use of your credit and debit cards used for Apple Pay (iPhone 6 and iPhone 6 Plus). See Apple Pay on page 130.
Erase iPhone: Protect your privacy by erasing all the information and media on your iPhone and restoring it to its original factory settings. Erase iPhone also removes your credit and debit cards used for Apple Pay (iPhone 6 and iPhone 6 Plus). See Apple Pay on page 130.