A people pleaser usually is from the high context, and implicitly by accepting to help there is an expectation that the person asking is at least just not merely exploiting them for personal gain and at least aiming to increase the collective value.
Once you have identified what type of culture someone tend to follow. It becomes easier to say no to them because you know what to expect. It also allows you to know when to say yes even when the presented deal doesn't seem immediately rewarding because it opens some new interesting paths often leading to hard to anticipate long-term win-win situations.
As a bonus, identifying their culture will allow you to communicate more effectively.