If you're just wanting to cancel Cox Complete Care, that's a simple matter of telling them such and they'll remove it. The problem is, if you had a technician come out for service and the charge for the visit was covered by Cox Complete Care you're obligated to reimburse Cox for that service charge if you proceed with cancelling Cox Complete Care.
It might be cheaper for you to keep Cox Complete Care at $10/mo. for 3 months ($30 total) vs. paying the $75 service charge for the technician's visit.
Edit:
I'd also suggest emailing the moderators here at cox.help@cox.com and include your account info so that one of them may review your account with you and explain exactly what happened and what your options are for resolution.