英文問題，高人請進

I think it's better to change the word [selfless] to [considerate (adj.)]:

Be a good and considerate citizen. No smoking please.

[selfless] generally means [無私/ 忘我]; while [considerate] means [懂得體諒別人/ 不會只顧自己利益]

Be a good and responsible citizen (person), no smoking pls

Be considerate, no smoking pls

For a disinterested citizen, no smoking please.

Taking others into consideration, no smoking please.

Being a caring citizen, no smoking please.

Empathizing others, no smoking please.

Be accountable to the public, no smoking please.

Regarding others, no smoking please