Forcing players to wear playermodels

So I am currently a developer on a server but I’m stuck with playermodels.
I have added them to the pointshop so players can buy them, but some players actually go into singleplayer and pick the playermodel they want there.
Then they rejoin the server and have the model without owning it in the shop.
Does anyone know how to fix it so everyone who joins defaults to a certain model?

Thanks in advance

What gamemode?