I can identify the account sending out the spam, configure CSF/Exim to auto-block abuse, and clean the site affected.
I quoted a Wordpress cleaning, since that's the most common.
Relevant Skills and Experience
I run a web hosting service based on WHM and have it locked down against spam.
Proposed Milestones
€40 EUR - Identify site affected, configure CSF/Exim to clamp down on it.
€50 EUR - Clean Wordpress site.