Hi there and welcome, playerId is indeed like @d0p3t impossible to “lock” and even “dumb” to do IF it was possible
since that would also mean that you would make it so that once you’ve had 32 (or 64 on 1S) players no one new can join anymore, OR you would have to make a complicated pool blablabla
but all in all: not possible and
However you can easily check with the function/snipped bellow on identity’s of players
i’m using this code for a long time and working great, it originates from:
function GetPlayerIdentifierFromType(type, source)
local identifiers = {}
local identifierCount = GetNumPlayerIdentifiers(source)
for a = 0, identifierCount do
table.insert(identifiers, GetPlayerIdentifier(source, a))
end
for b = 1, #identifiers do
if string.find(identifiers[b], type, 1) then
return identifiers[b]
end
end
return nil
end
then using this line for example:
local PlayerLicense = GetPlayerIdentifierFromType("license", source)
This will give the ‘player/client’ unique license identifier, you can also get the IP from it and even compare the playername with it 
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