How can I make a seat where you can't get out by jumping?

Ok so, I tried to find the guy who originally made this script but I couldn't

Basically, as "ym5a" said, use Seat.Occupant

Here's what you're looking for

local occupant = nil -- Nobody is sitting on the seat by default.
chair = script.Parent.Parent

script.Parent:GetPropertyChangedSignal("Occupant"):Connect(function()
    if script.Parent.Occupant then -- If there now is an occupant
        occupant = game.Players:FindFirstChild(script.Parent.Occupant.Parent.Name)
         --[[
             The occupant variable is now referring to the player that is occupying the seat.
             Note that the Occupant parameter returns the player's humanoid, not the player himself.
        --]]
    elseif not script.Parent.Occupant then -- If there is no occupant
        occupant = nil
    end
end)

while true do
    wait()
    if occupant then
        occupant.Character.Humanoid.JumpHeight = 0
        occupant.Character.Humanoid.JumpPower = 0
    end
end

Insert that code in a script inside the seat, if you wanna learn to do it yourself just try searching how to get the player sitting on a seat.