  An oriented compact 4-manifold $V$ with boundary $S^3$ is called a positon
(resp. negaton) if its intersection form is positive definite (resp. negative
definite) and it is simply connected. In this paper, we prove that there exist
infinitely many knots which cannot bound null-homologous disks either in
positons or in negatons. As a consequence, we find knots that cannot be
unknotted either by only positive crossing changes or by only negative crossing
changes.
