Malicious file removal

My domain is blacklisted at spamhaus.org and I cannot send email to a client.

According to Spamhaus, my WordPress site contains a malicious file.

The malicious files can be found here: hXXp://grandesign.com/wp-content/uploads/gravity_forms/1-ac72a0676134f93b1a5d294342822e91/tmp/ajax.php

Any idea how I can take care of this?

Thanks very much in advance