I received like 5 texts from them about installing the app and earning rewards. One of them said I could reply 'STOP' to stop receiving message. They sent me one more text stating I was removed and I haven't seen anymore texts from them since 1/1.
I would guess that FreeUp is sending these messages to people on the Free and paid plans alike, although members on the Free plan cannot do anything with rewards earned until they upgrade to a paid plan. It just shows how important having members earning rewards is to FreeUp's bottom line, I suppose. @drysun reporting of the "STOP" option seems to be the only way of opting out of these messages, so far, but hopefully others will see this option ,too.