I am running a proxy server in linode.
Now, there is some one use our proxy to spam post,
Is there a software to block this behavior?

Thank you.

Just require authorization/authentication so that only the required people can use the proxy, and not everyone. How this is done depends on your proxy software, but squid for example does have authentication possibilites.

our proxy server is not free.
need authorization.

Then check the logs to see what user visited the sites that were spammed so you can figure out which user's account was compromised.

My proxy server now have no log before.

Any software can be used to block spam post later?