Since Purism chose to suspend my account on their forums and delete my posts which provided customers with instructions on how to report their violation for not providing refunds to the FTC, I'll post it here again, with the hope that its helpful for some of you.
Go to https://reportfraud.ftc.gov/#/assistant
Select: Online shopping
Select: A problem with an online purchase or sale
Click: Continue
Check: I never received a correct refund
Fill out the rest and provide information about your individual case
Purism can then be fined for up to $46,517 per violation, because they're by law required to provide refunds for canceled orders within seven working days[*]. As I understand these reports can be made regardless of the country you're in.
[*] https://www.ftc.gov/business-guidance/resources/business-guide-ftcs-mail-internet-or-telephone-order-merchandise-rule