delete my account - Remove your info from your profile and click Save on the main My Profile screen. Go to WhosHere > Setup and click sign-off. Within 45 seconds your account will be removed from all search results and you will be blank to anyone that you have chatted with in the past. Then just delete the app from your iPhone.
If you are going to sell or recycle your device, you should also consider restoring it to Factory Settings (support.apple.com) to delete all personal information from the device.