Answer:
Hack they're account. Just kidding.
Stalk them relentlessly on a new account. Just kidding.(I , might work :P)
You can not really force a hacker off your account. If you do and get your password, your stuff might be all gone and loaded into the hackers main. Sorry man, it may be wiser to get a new one.
I your persistent, then report to Jagex, then show proof that your account is yours, then they'll hopefully reset your password to your liking. If that is successful, make questions and a password extremely personal to you so no one but you will know the answer.
IF you get your account back say hi: Zero_zx12