I don't know why you couldn't, because I just tried it, first from the administrator account logging out a user account with admin privileges, then just the reverse, and it worked both times.
Yes, you can log off a user from Task Manager. The tutorial below can help show you how to. You have to click on Show processes from all users to run Task Manager as administrator first to be able to though.
Look at the note in step 3 in the tutorial link above. You will need to do this from within an administrator account, and click on the Show processes from all users button in Task Manager first.
System Manufacturer/Model Number Custom OS 7 home premium CPU P4 2.8G Motherboard D865GLC Memory 1Gb Graphics Card GeForce 6200 Sound Card SoundMAX Integrated Digital Audio Monitor(s) Displays CMC 19"
PSU Therm 430w Case Metal Cooling Intel standard Hard Drives Maxtor 2F040L0
SAMSUNG SP0411N
WD3200AAKS
WD5000AADS