Deny a Sql Server user from a specific hostname
And, there several different developers which developes that CRM web
  app. They know the user name and password from the web config file
  they develope. Some of them can connect to my database with that user
  and execute some DDL and DML commands. And i want to block them by
  their client hostname. If a developer connects to db from his/her
  machine with the user credential, i want to block them.


I don't think you should try to fix it that way.

Both pc names and IPs can be changed at will, even static IPs could be changed if wanted/needed. Which means you'd have to maintain an up-to-date list all the time.
As a dev, if you really wanted to do something, remote to another VM, execute there.
I mean, there's probably plenty of security loopholes they can find if they really wanted to do something.

Finding a solution to do what you want, would probably only make it a hassle for them, but not fool proof.

I think you'd be better off trying to find a security solution for this. If one of the devs should not be able to do DML statements, don't give them a login that has access to it. Does the user in the config really need all the permissions it has now? Perhaps you can give less permissions to that user, and give the people who need more permissions an extra role, on their own login.