is there a way I can disable a players movements in a server script?

Just a mock-up code (untested since I'm on my phone). But the Guts of this code should be what you want.

Combining Both comments from StayMarkus(WalkSpeed) and BearGamesZ(HumanoidRootPart.Anchored)

HumanoidRootPart.Anchored

local Players = game:GetService("Players")
local Freeze = false 

while true do
    for k,plr in pairs(Players:GetPlayers()) do
        local Char = plr.CharacterAdded:wait()
        if(Char ~= nil) then 
            local HRP = Char:FindFirstChild("HumanoidRootPart")
            if(Freeze == true) then 
                HRP.Anchored = true
            else
                HRP.Anchored = false
            end
        end

    end

    wait()
end

Humanod.WalkSpeed

local Players = game:GetService("Players")
local Freeze = false 

while true do
    for k,plr in pairs(Players:GetPlayers()) do
        local Char = plr.CharacterAdded:wait()
        if(Char ~= nil) then 
            local Humanoid= Char:FindFirstChild("Humanoid")
            if(Freeze == true) then 
                Humanoid.WalkSpeed= 0
            else
                Humanoid.WalkSpeed= 16
            end
        end

    end

    wait()
end

Both bit of code are untested but it should show you how to do this!. Hope this helps!

There's actually two ways and the first one might not work.

You could try this way by simply adding a server script in the ServerScriptService.

local SP = game:GetService("StarterPlayer")

SP.CharacterWalkSpeed = 0

-- And if you want, we can disable jumping too, but just delete this part if you don't.
SP.CharacterUseJumpPower = 0

OR

You can do it this way:

local Plrs = game:GetService("Players")


Plrs.PlayerAdded:Connect(function(player)

    local char = workspace[player.Name]

    -- Now we got the character, we can find the HumanoidRootPart.

    local root = char:WaitForChild("HumanoidRootPart")

    -- Anchoring the root part will stop the player from moving, or jumping.
    root.Anchored == true
end)

I hope this helped! Mark as solution if it did :D