Create a new "role" corresponding to users with (say) 150 points, assume you cal the role "UsersWith150Points".

Then any time a user gains points, use a rule to check if the user passes the 150 points. If so, have rules grant access to the "UsersWith150Points" role. Similarly, if a user looses points, also use a rule to check if user goes below the 150 points. If so, have rules remove access to the "UsersWith150Points" role.

Secure the access to the flag to users with role "UsersWith150Points".

With that, you have everything you would need.