What he said.
To do this, go to https://my.roku.com/account
and scroll all the way down.
As suggested, look at the devices listed. If Netflix, or any app, is added to any one of them, it will add to all of them after an overnight refresh, or after a manual refresh. Likewise, removing it will remove from the others, after a refresh.
If you see any devices you don't recognize, Unlink them.
If there are no unexpected devices, add a PIN to your account via the PIN Preference setting, selecting "Always require a PIN to make purchases and to add items from the Channel Store." Of course, you may want to do that anyway if anyone else has access to your Roku, or access to any Roku tied to your account.