Basically, you just wanna change the password and email (obviously) and you also wanna check if they have a Facebook account linked to it, if they do you wanna link yours to it or just de-link it. Hope I helped, PM if you need any help.
The only ways I know are to change the email/pass, remove any and all alts, unlink every other social network, and, deauthorize all apps with permissions to the account.