Banning a specific player is a real easy job on jass.
Try this and tell me how it works for you.
Under the locals of function main
Code:
call banPlayer("Enter Name Here")
under endglobals
Code:
function banPlayer takes string nameToBan returns nothing
local integer index=0
set nameToBan=StringCase(nameToBan,false)
loop
exitwhen index>11
if StringCase(GetPlayerName(Player(index)),false) == nameToBan then
call CustomDefeatBJ(Player(index),"You have been banned from this map")
endif
set index=index+1
endloop
set nameToBan=""
endfunction
I wrote that on the spot using my mobile so it might have a few syntax errors, so just paste this on JassCraft and press F9 and tell me the errors it gave you.
All in all it should work, if you want several people banned you can just call the function again witha a different name each time.
The function is NOT case sensitive (makes it more helpfull)