how to stop ping? CTRL C is not working

A single control-C will not do it and it may take multiple attempts. Even then, control-C may not interrupt the ping process, and then you may need to enter control+\ several times. When that does not work, open a new Terminal window and enter the following:

pkill -9 -ix ping

This will kill ping without question.

It is a good idea to start ping with a safety net. The following will cause ping to quit after 300 seconds regardless of what it is doing:

ping -t 300 foo.com

Note that entering ⌘+Z in the Terminal does nothing except make a noise. Ping continues to run regardless. Entering control+Z only suspends ping, but does not kill it, unless you know the job number:

"Command " on keyboard and Z