How does Sweet Mask (A-Class rank 1) plan to stop unworthy heroes from moving to S-Class?
As mentioned in this answer, 


  When heroes are ranked #1 in their class they can choose to stay at the top of their class or start at the bottom of the next class.


There is no other way than being #1 to rank up. Therefore, as long as you are #1, no one in your class except yourself can rank up to the next class.

Sweet Mask is "supposed" to be way too powerful for the A-Class (from what he says, as we never saw him fight), so there is no way anyone in the A-Class would ever be stronger than him. Therefore, no one can take over the first place of A-Class as long as Sweet Mask doesn't rank up.

The only limitation of his system is that he can not decide on who can directly start as a S-Class, as it is only decided on the tests. This is why Sweet Mask could not influence Genos' rank