Hi,
Go through the section "Deleting and Reorganizing Trace and Log Files" mentioned in the below link
Tracing und Logging - Search and Classification (TREX) - SAP Library
In the mean time you can try nullifying the log file.
/dev/null command will work on Unix systems for Windows you can try.
C:\> type nul > file_name
Better create a log file of same name and rename the old one. And than you can try either deleting/zipping the file
Regards,
Prithviraj.