Including/Excluding Specific Segments

What is the best way to suppress people who already made a purchase and do not need to get additional emails? 

Many customers create contact lists or queries for their segments and then create a new query that uses the criteria "query" and then state "is" or "is not" in that other segment query.  Similarly, you could select the criteria "lists" and use "is in contact list" or "is not in contact list" for inclusion/exclusion of contacts.

Segment Queries: 

Gmail Users = email ends with gmail.com

Opt in this year = opt in after Jan 1st, 2018

New Query using "Is in contact list" and "Is in Query":

For example, if you made a query of people that purchased a product and then make another query that says they aren't in that query, you could email and exclude that segment. 

Alternatively, you could also export a query and import it as a suppression list, however if you did it this way, you could email someone that purchased between the time you exported and the time you sent.  The query strategy would be realtime as the queries would calculation at the time of send.