I have an app that i specified target devices as iPhone, not iPad, not Universal. Something strange happened when i look at the app information, on compatibility section, on App Store: it is said that the app is Compatible with iPhone, iPad and iPod touch.
How can I remove the compatibility with iPad?
Any iPhone app can be run on an iPad in an emulation mode. There is no (official) way to prevent this.
To test that your settings are correct, go to the App Store and search for your app with "iPad apps" set. If it doesn't show up, you're good to go.
You can still download this app on iTunes, or set the App Store to search iPad and iPhone and install it on an iPad, but it runs as an iPhone app on the iPad.