Hey Kirshia, thanks for the inquiry! Sounds like you're trying to build a Do Not Contact list that would also help prevent spending/enriching them further if you've already added them, is that right?
If so, currently, there's not easy way to exclude them from future searches but it's on our roadmap to be able to Exclude any type of lists from Find People.
A workaround is to import the list of contacts you have into a separate Clay table with any identifier value you may have for them. For example, if you have people's LI URLs, you could easily crossmatch the Used In Smartlead table vs the new list of people to check if the LI URL is found to then set up conditional formulas so existing records don't get enriched or contacted.
Here's an example of how to setup a lookup for DNC (a bit old UI but the concept/idea could still help) - https://www.loom.com/share/80d613c10d5248f6bfc662cc95c8023d
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