I have a question on what i can do if i stop a player from spawning?

Unfortunately, I have run into the same issue as well.

I just allow the characters to spawn, and then when they're done spawning I just destroy them. The guis load, and it achieves the same effect.

No, StarterGui does not clone it's children to PlayerGui if the character doesn't spawn. But, you could easily spawn a player in a black box, then teleport them when you're done.

EDIT: Teams

A simple solution to this would be to kill the character, but this is a method I dislike, when you could just as easily teleport the player to the spawns, like so:

-- this script needs to be a LocalScript inside the button that you click to spawn
local spawn = game.Workspace:FindFirstChild("RedSpawn") -- you can have multiple spawns named this, it will choose the one at the top of the list
local p = game.Players.LocalPlayer
local button  = script.Parent -- button inside gui that you click to spawn

button.MouseButton1Click:connect(function()
    local char = p.Character
    if char then
        char.Torso.CFrame = spawn.Position + Vector3.new(0,7,0)
        button.Parent:remove() -- closes gui when clicked, in this case, gui is the parent of the button
    end
end