I once had to locate my phone from my ex-boyfriend's cell phone. Now he is constantly locating my whereabouts with Android Device Manager. I have removed my device from his list of devices on his account. However, my device still shows up when I go to "find my phone". I do not have access to his cell phone or computer as he has moved 2000 miles away. I have tried everything I can think of online. Please help.
It's a really simple solution: change your Google password. This will lock him out of your Google account.
If he never had access to your Google account and he is tracking you from another Google account you are signed into on the phone, go into settings -> accounts and remove the offending account that gives him access to you phone's location.
If neither of these solutions work, you can disable the Android Device Manager for your device entirely. Go to settings -> security -> Android Device Manager -> Remotely Locate this Device / Allow remote lock and erase. This is a foolproof way of fixing the issue, but will remove your ability to use the Android Device Manager entirely and is a new setting added in Marshmallow, so you may not have it. The first 2 options, however should fix this.
As a side note, the fact that he is still tracking you with this against your will could give you legal grounds for stalking depending on where you live.
Definitely add something about 2-step and make sure not to hook it up to phone number.
This is definetly something to be added, and I'll start out by explaining what it is since while it is thrown around as commonplace today, not many people really understand what it is. 2 factor authentification adds a second layer to the sign in process that makes it significantly harder to log in if you are not the account's holder. After entering the correct password, the application sends a message to the account holder with a second one time use code to log in. This code prevents others from logging in as you. However, this isn't terribly secure as the usual route of sending the message is with a text which can be easily intercepted. Instead, use
Google Authenticator, which stores the second code in your phone. I won't go into the specific of why this is more secure. Just know that it is. It's an app you can download that will guide you through the process of setting it up. The second code will always be inside that app. When you need to sign in to a computer for the first time, it will ask for the second code. Open the app and type in the second code the phone is currently listing. Note that it changes every 20 seconds, so if you don't have enough time to type it in and hit enter, wait for the timer to reset.
I would go a step further than the great answer by @Ethan_Z...
1) Change your Google Password and enable 2-Step verification and sign out all other sessions
2) Backup your device
3) Delete all Google accounts from the phone
4) Perform a full factory reset
5) Setup the phone again like a new device and do NOT restore apps automatically from the Play Store during the Setup Wizard unless you absolutely sure what each and every app is
Unless your devices is rooted, this will virtually guarantee that you will no unauthorized person is able to track your device via ADM or other means.