Code:
on *:text:*!start*:#: {
  if (($1 == %c $+ !start) && (!$var($+(%,$nick),1))) {
    set $+(%,$nick) 100 | msg $chan $nick starts with 100 points.
  }
  else {
    msg $chan Sorry, but you already had your 100 points.
  }
}

$var($+(%,$nick),1) breaks down like this $+(%,$nick) = the var name of the variable holding the nicks amount, $var( ,1) this then tests if that variable exists (even if its value is zero) or $null, if it exists its true, if it doesnt exist its false (it actually returns the variable name which makes it true, but if it dont exist then it returns a $null which is the same as false for a IF)
The ! at the front reverses the condition, since we want to go in only if its false (no varable %nick exists)


Now you could be meaner and use this
Code:
on *:text:*!start*:#: {
  if (($1 == %c $+ !start) && (!$var($+(%,$nick),1))) {
    set $+(%,$nick) 100 | msg $chan $nick starts with 100 points.
  }
  elseif ($calc(1 * $($+(%,$nick),2)) isnum 101-) {
    set $+(%,$nick) 100 | msg $chan Since you insisted $nick $+ , your score has been reset to 100 when it was $v1
  }
  else {
    msg $chan Sorry, but you already had your 100 points.
  }
}

Same thing, but if u do a !start and u got over 100 points it resets ya to 100