They are looking for spending on cash equivalents i.e. Prepaid gift cards and money orders which can be liquidated for cash. They are also looking for people who buy things to meet minimum spending, get the bonus, then return the item after they get the bonus. They call that "returns".
They especially hate people who do "returns", get the bonus, transfer the bonus out then close the account to get a refund of the annual fee.
Someone I know met the minimum spending organically but then made the mistake of downgrading her platinum card which has an annual fee of $450 to a green card which has a fee of $95. After she downgraded, they clawed back her points because she received a prorated refund of her annual fee..