1) I'm assuming you have Windows, since you posted in Windows 8 section
2) I'm only good to bounce ideas off of, I'm not a security expert by any means
I'm assuming you already tried restarting the laptop.
I'm assuming you already ran a virus scan, which suggests (but doesn't absolutely mean) there is no infection (Also assuming the scan came up negative)
-If the other account is connected via the web, disabling wifi / unplugging ethernet might force them to disconnect.
-If you have an administrator account, change the password for the new account, log into it then log off.
-{Extreme option} Format your disk. You'd want to save all of your information, but if it's a virus of some form (such as a Trojan), you may accidentally save it with the rest of your important files. Having a backup, even one that may potentially be infected, is better, in my opinion (as a non expert) than having no backup at all.
If I think of more things, I'll suggest them.
Once again, I am NOT a security expert.