What app are you using? Some email apps attach a default signature. Some let you turn it off, some only allow that in paid versions (though those normally include the app name in this signature).
This will be a feature of the particular email app, not of the operating system or anything else. So if all else fails you can just use a different app. But if you tell us what app you are using hopefully someone will be able to tell you where you can find the setting.