In my game, players can use the camera to see through walls! Help?

Yes, it's normal. Nothing you can do except maybe make it first person? Although that inst needed really.

You could, add another part to each wall, slightly smaller. So your camera hits the Major wall, and sees the inner wall.

function GetObjects(par,parameter,tab) -- My find function --
    if not tab then tab = {} end
    for _,v in pairs(par:GetChildren()) do
        if parameter then
            if v:IsA(parameter) or v.Name == parameter then
                table.insert(tab,v)
            end
        else
            table.insert(tab,v)
        end

        if v:GetChildren()[1] then
            GetObjects(v,parameter,tab)
        end
    end
    return tab
end

for _,p in pairs(GetObjects(Workspace, "BaseParts")) do
    if p.Anchored and not p.CanCollide and p.Size.magnitude > 4 then -- Not needed but logical.
        local InnerWall = p:Clone()
        InnerWall.Parent = p
        InnerWall.FormFactor = "Custom"
        InnerWall.Size = p.Size - Vector3.new(0.1,0.1,0.1)
        InnerWall.CFrame = p.CFrame
    end
end

Line 20 isn't really needed, but it'll cut down on unnecessary brick count.