I'll have to hazard a guess as to what this well known Jamaican proverb means. Shoot means to put out fruit. Plantain is a cousin to bananas, it can be fried, boiled, roasted or used as a filling in pastry - not usually a pie, more likely plantain tart. It is sweet when ripe and has a mellow flavour. When green it is starchy and bland and the flavour is usually enhanced with salt, pepper, onion or garlic.
After a plantain plant bears and you harvest the fruit, you generally cut the plant down because it will eventually die anyway. If you are too lazy to cut the plant down, you are merely encumbering the ground and having this useless plant compete with productive ones for space and nutrients.
Now, to map an interpretation of the proverb directly to this, I'd have to say that when you feel you've lived long enough, just put on your best show. But strangely that's not how I've heard this saying used. Here is a context in which it might be used - young Susan is blossoming, vivacious and confident, maybe laughs too much in her granny's mind. Granny is worried that some man is is going to come along and pick her cherry (and as far as Granny can see) blight her whole future. So Granny seeing this, hrumphs and issues the cryptic warning; when plantain want dead him shoot.
Or here is another scenario Marlon is extra energetic today and he is up to all sorts of mischief and making a complete nuisance of himself. Pappa is quickly losing patience and will reach for a strap in a moment, but before he does so he prepares Marlon; when plantain want dead him shoot. If Marlon gets the message he will stop the tomfoolery.
Do they say anything like this in your neck of the woods?