Recurring malware

Google detected malware on one of my sites, after an awful couple of days I encounter the malware on my files and deleted it. The site worked fine for one day before it came back. Then noticed that the code was also present on 2 other sites, those 3 sites are hosted in ANhosting, their support guys have been useless only avoiding the subject.

I know where the code appears and I’ve been delete it 2 or 3 times a day.

Im so lost with this, how can I get rid of this malware thing for good and how can I prevent this to happen again?

Thanks in advance!