I have tried stopping download manager . But as soon as I stop it , it starts again . I cannot even clear its data and cache . Tried rebooting the phone, it doesn't work.
I have an Asus Zenfone 2 still running Lollipop, and I've had the same problem for a long time. Force-stopping the Download Manager app and clearing the data usually works, but I recently stumbled upon a new way to cancel the download, making sure that it is permanently cancelled and doesn't pop up again annoyingly.
It merely involves going to the Storage section under Settings, and clicking on the Downloads tab. This brings up a list of previous and ongoing downloads. From here, long-pressing on a download lets you cancel it.
Hope this helps.