WordNet (r) 3.0 (2006):

draw the line
    v 1: reasonably object (to) or set a limit (on); "I draw the
         line when it comes to lending money to friends!" [syn:
         draw the line, draw a line]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "draw the line":
   analyze, balk at, be unwilling, begrudge, bound, condition,
   confine, contain, contradistinguish, copyright, demarcate, demark,
   differentiate, discipline, discriminate, discriminate against,
   distinguish, divide, grudge, hedge about, limit,
   mark the interface, mind, moderate, narrow, not care to,
   not feel like, object to, patent, pick out, qualify, red-bait,
   register, restrain, restrict, scant, screen, screen out, segregate,
   select, separate, set a limit, set apart, set off, sever,
   severalize, sieve, sieve out, sift, sift out, sort, sort out,
   specialize, split hairs, stint, straiten, subdivide, subtilize,
   winnow, would rather not