The iPhone's clipboard keeps track of the last thing you copied using the device's "Copy" function. Unfortunately, Apple failed to include a simple way to clear the clipboard. Even after you reset the device, the clipboard will still contain the last thing you copied. However, you can erase the last thing you copied to the device's clipboard by simply copying a blank space.
Tap the "Messages" icon on the device's home screen. Tap the "Compose" button in the upper right corner of the Messages screen to start a new message.
Tap the space reserved for the message's text to place the cursor there. Tap the "Space" key to make a space in the text box.
Tap and hold your finger on the space you made in Step 2 for several seconds. Release the space and tap the "Select All" option from the context menu that appears.
Tap the "Copy" option to copy the blank space to the clipboard and clear out anything else that was on the clipboard before.