Automaticly Getting unwanted files in my system.

Hi It's a very critical issue, which is am facing from past 15 days. i have placed too many restriction to my website.
Disabled file permissions.
Added Query String rules in .Htaccess
Add IP blocker.
Removed image.php
removed timbthumb.php and many more.

Now i got frustrated from these issue, since i transferd my website at your server facing issues.

Now i need to know whats happening and who is doing this ?

Right now i have a file at my public_html, file name : wpkb.php.
I need to know from where this file came to my server and who is doing this ??

I blocked many ips from ip blocker but still happening the same, so am not sure is this your are someone else.
Daily i remove 20 unwanted files from server.

Please check this issue and try to resolve this asap.
Please ad my email to the system when you reply to this ticket, as i am developer who is working on this website.
My Email : [email protected]

My Login email address: [email protected]
Regards,
Praveen