No, -LA isn't related to any feminine.
SMETTILA is an admonishment to someone who is annoying you, so you
So you say "Smettila"
It can be translate with "STOP IT"
It is always referred to a second person (you)
That's from the verb SMETTERE (to stop)
If you need to tell this to two or more people, then you use the plural form: it is SMETTETELA
PIANTALA it's a synonym and it's made from the verb "PIANTARE" (to plant) but in this framework isn't referred to anything about gardening ;)