。
I'm quitting smoking after this last one.
Literal
I [topic-は] this-only [with-で] smoking-quit.
これっきり ('this is the last time, never again') is a colloquial expression made of これ + っきり (only, just). Combined with で it means 'with this as the limit / after this.' 禁煙する ('to quit smoking') is a noun+する verb. A characteristically resolute statement.