Remove spamming malware& find the cause and fix it on my server

Hello, Several wordpress instances on different domains (on the same server) have been compromised and they keep sending spam outside using the domains (for now seems like 3 domains, therefore, 3 wordpress installations). I need you to find all the files (php scripts etc) which are used for spamming and remove/fix them throughout the server, then check everything ( e.g. all the wordpress instances) and find all the causes of this compromise (and fix them or explain me how to fix). this inludes checking everything, plugins, themes etc. Please also let me know on what you would need, I would like to give FTP access and WP admin access (although WP access is currently limited since the server admin has limited the access to the domains due to outgoing spamming activity). I will not give you cpanel pwd, in case you need something done there, you will have to tell me. Access to logs can be delayed as well since I am not the owner of the webserver. BUDGET: $20 ($10 for removing all the files and stopping spamming, then $10 for finding the cause and fixing it), bids above of it will be ignored.