Can bad potato salad be made into something good?
Microwave it (or bake in the normal oven in a casserole dish), add salt to taste, perhaps some butter or cream, weird mashed potatoes? Will depend rather on what, exactly is in this "deli potato salad" (which varies IME from potatoes and dressing to potatoes, many crunchy veggies, sometimes eggs, etc., etc. and dressing.) I've had quite decent versions that fit the former description, and some of the latter were terrible...

If the potatoes are adequately undercooked and you want some serious self-penance for buying it, you could possibly scrape the dressing off them and reboil to finish cooking, with salt, while adjusting the dressing (or pitching it, but mustard might save it if you like mustard, and garlic is always an option at my house.)