We deliver installs from real users, and can not fully control behaviour of users in reward app.
But be sure we do our best to prevent bad reviews.

We track all reviews our users write on google play and if user leave bad reviews with 1-3 star or review text contains forbidden words (for example, mentioning that he was paid for the installation), we send notification to user and ask to change or delete review.

Most of user change review to positive after that, because we do not pay them until not change review.

Of course we can not protect on 100% your app from bad reviews, but we do our best.