Hi Jess,
First of all please check already booked approved hours details in CJI3 report like where they are booked and then try to cancel the approve hours.
Please find the below link : -
http://wiki.scn.sap.com/wiki/display/ERPHCM/Cancelation+of+Time+Sheet+Data
http://scn.sap.com/thread/1112145
Regards
Shashank