I think cancer cells are making themselves more cancer already, I mean, mutating more, getting broken more, getting out of control more, spreading more... What you need to do is to either fix them, or kill them. Killing sounds easier.

But thinking about it now, perhaps if you could specifically find and broke them to the point which they can't function at all anymore, that might work too. But still, if you could find them, why not just kill them?

The Newcastle virus stuff which /u/Med_vs_Pretty_Huge has mentioned above sounds like something you may be interested in. In wikipedia it says that it prefers infecting and killing cancer cells more than they do ordinary cells, somehow.