I bought the premium version of the Todoist app for Android, but sometimes I still get ads at the bottom. Not always, but like 50% of the time.
unfortunately we don't offer any native Android apps right now, but we're currently working on it. All available Todoist Mobile apps for Android are third-party solutions.
Please contact the developer of this app to clarify whether using Todoist Premium is supposed to solve this issue. Sorry for the inconvenience.
For now, you can also try out our Mobile site accessible through your phone's browser: http://todoist.com/m/