Words or phrases that indicate the opposite of a goal or target, meaning This is something to avoid, as it opposite of my desire

Solution 1:

Perhaps simplistically, the opposite of target is nontarget:

[Merriam-Webster]
: not being the intended object of action by a particular agent
// effect of insecticides on nontarget organisms
// More than 200 deaths were nontarget animals, including foxes, opossums, raccoons, skunks and a bear.
— Sophie Lewis, CBS News, "Trump administration reauthorizes use of "cyanide bombs" to kill wild animals," 8 Aug. 2019

So, in one of the example sentences:

Conversely, my business's nontarget customers are Millennials because they complain and buy very little.