Firstly are you sure its not a family member winding you up via a mobile phone app or openwebif.
a couple of things you can do.
Make sure you have changed your password in
setup > system > network > password setup
and dont use common words. Use random letters and numbers.
and
plugins > openwebifconfiguration
Enable http Authentication ... yes
that will mean the user would have to type a username and password to access via a browser.
Username: root. Password: whatever you changed it to
