For the record, cause I came looking myself, you can stop the CPanel backup process by logging in via SSH, and doing a
lsof |grep cpbackup
You'll get some feedback in the running process like this:
cpbackup 4977 /usr/bin/perl
cpbackup 4977 /lib/tls/libc-2.3.2.so
cpbackup 4977 /lib/libnss_files-2.3.2.so
cpbackup 4977 /lib/ld-2.3.2.so
cpbackup 4977 /usr/lib/perl5/5.8.1/i686-linux/auto/Socket/Socket.so
cpbackup 4977 /lib/libdl-2.3.2.so
As you can see, the PID for the process dominates the result. Now you can do a:
kill -9 <pid> as in
kill -9 4977
then just do a "top" and kill the latest backup PID
Hope that helps someone,
Mike