Hi, i have experience with solving similar issues, and i would like to offer you my services. The only way to solve this is to check all files for malware, and resoter them to default state (provided by Wordpress and other parties). Please note Wordpress should be pretty much secure, however some plug-ins have poor or no security inside (they may be altered by me to close the doors). If you have any question or more information, please don't hesitate to contact me by private message. I'm looking forward to our cooperation.