There are a few different ways - the simplest is probably to maintain an exclusion list as a custom audience and adding them there, being sure to add as many details as you can to try to exclude the right person. Bear in mind that this list needs to be at least 20 people. I’m not sure what they are threatening you about, but I’m pretty certain the onus falls on the Facebook user as part of the t&cs to receive relevant ads - that sometimes being through lookalike audiences and therefore you as a company do not have control over who you are targeting. When they see an ad they can mark it as irrelevant. Though, I do appreciate sometimes that’s hard to get across to an upset (non) customer and may create more discourse on the topic.
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