This is not a "Grow a backbone" issue. What's next is that some snitching is going to happen. The witless seeing you buy the "sun-worship" tree is going to snitch to Brother Hounder, because they are programmed to believe that if they don't, they also as good as bought one themselves. At this point, Brother Hounder is probably going to have a little meeting with the other hounders about this--it is riskier if you are still active or were within the past 6 months, but even inactive witlesses out 10 years or more are still bound to be hounded. Sometime later, the hounders are bound to show up at your door or start calling you up.
It is then that you need to "grow a backbone". Your options are to blow them off (not answering the door or the phone) and hope they get tired of wasting their time, tell them point-blank that you intend to keep the sun-worship tree no matter what, or cave in and throw it away and "repent". If you blow them off, you are going to be hounded but have the hope of wasting their time and resources that otherwise could have been used to tend to other matters. If you are point-blank, you will end the hounding but will probably be disfellowshipped quickly with a minimum of wasted time. Either way, you are not giving into their demands that you get rid of that Christmas tree. The only "caving in" option is to listen to whatever the hounders tell you, do as they tell you, and return to the boasting sessions. That will give them the control they seek, and you will probably be reproved.
Or, you could (in lieu of just telling them that you intend to keep the tree) simply announce your disassociation. That will probably be the end of the matter, they will not be able to control you, and you get to keep the Christmas tree. You will be cut off from all association, and it will prevent you from wasting any more of their time. Either way, I don't recommend complying with the hounders.