There is no 'rule', but you can find definitions of what is considered abuse on the SFR discord, e.g.

Bid bot abuse
Compared to similar posts in the same category, the bought votes increase the rewards of the post above 50% of similar content. Blatantly overvalued bad post.

Think about if you are adding value to Steem or just being greedy. If you are not attracting real votes and comments then you need to re-assess your strategy. There's a limited amount of Steem to go around each day and we like to see it go where it is deserved. SFR can only remove part of your rewards, but that may be enough to make it non-profitable for you.