You sit the relatives down, tell them that you love them, but going forward you will no longer be exchanging gifts with them.
In my family's case, I told them that we have an abundance of material things, and no real wants that we haven't satisfied easily, and they are also blessed with more than they could ever want or need, so going forward I plan to donate to worthy causes to help those less fortunate in their name, and ask that they do the same for any special occasions like birthdays or Christmas. If they complained, I just explained again that I have no wants and so anything given would be sold or donated, and my love for them shouldn't be measured by the cost or type of gifts I give them.
So TL/DR: tell them how you plan to go on going forward; no material gifts given, any given to you will be sold/donated. Politely but firmly without trying to convince them that your way is the right way for all, but this is how you will be handling things on your end in the future.
You are an adult and as such, allowed to disagree with family members. You may have to face consequences like upsetting the status quo, or facing anger or disappointment, but their acceptance isn't on you. You just have to stay polite but firm about what you want to do, and do it.
Saying you have to convince them or can't do what you want to do isn't true; you just have to deal with the fallout in a mature, calm manner and remember - you are allowed to do what you think is right and best for your own household.