Try this: http://my.freedompop.com
(may not be a refund, though)
Login to the device you want to cancel and click 3 lines in a square in upper left to open menu.
Likely both SIMs are in same account. Might not want to gamble here to cancel.
Select Account Status - click RED Cancel Account.
Try to see if answering the questions does a cancel for you. Some people are asked for a password and the normal password is rejected as wrong. Do below instead if so.
Alternatively go to "Support" and enter a help ticket. Select "I want to Return my device / account" An Expert will offer to get you started. They will request the website ordered from, the date ordered, and the ICCID from your SIM. starts 89... and is 18-19 digits long. They will ask again for it because it is blocked on first support post. May be split into 2 lines on the tiny SIM.