Try checking the phone for viruses, malware, spyware, bad apps, etc. If you can't find the problem that way, then I suggest saving the personal stuff (photos, contacts, etc.) to a google account or a computer and then doing a "Hard Reset" on the phone to remove anything that could remotely be on it from them.
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press...