Tin and copper are in 3 different mods, 2 of which are currently in Technic and Tekkit: RedPower, IndustrialCraft² and Forestry. Forestry obviously isn't in. RedPower has a config wherein its tin and copper are normally disabled. I'd do that if you're finding them, and while you'll still see them in chunks already explored, new ones shouldn't have them. In theory.