You can log out from Windows store easily, Just click/tap the Settings charm while yo're using Windows store, There you can find an option "Your Account", Click it and log out from here.
By the way, when you log out from you PC Windows 8, you're logged out of each and every apps including Windows apps store. So, there is nothing to worry. In short way, You can follow these :
Open Windows store
Press Windows + I
Press Your Account
Press Sign Out and You're done!