How To Block iMessage Alerts From An Unknown Contact On iPhone, iPad, iPod Touch

Step 1: Open up the native Settings app on the iOS device in question. iMessage functions over a Wi-Fi or cellular data network, so the device could be an iPhone, iPod touch or an iPad.
Step 2: Select the Notifications entry in the second group of available options in the list of settings.
Step 3: Locate the Messages app from the list of apps installed on the device. If the current setup allows the Messages app to post notifications to Notification Center on the device, then it can be found under the ‘In Notification Center List‘. Alternatively, it will be towards the bottom of the list.
NOTE: For this process to have any effect, the Messages app is required to have at least one type of notification alert enabled, such as a banner or standard alert.
Step 4: Scroll down through the available Messages preferences until you get to the ‘Show iMessage Alerts From:‘ section. Make sure that the My Contacts Only option is ticked.
Step 5: The simple but effective process is complete. Only incoming iMessages from saved Contacts will be shown through notifications.