  The total spin of correlated electrons in a quantum dot changes with magnetic
field and this effect is generally linked to the change in the total angular
momentum from one magic number to another, which can be understood in terms of
an `electron molecule' picture for strong fields. Here we propose to exploit
this fact to realize a spin blockade, i.e., electrons are prohibited to tunnel
at specific values of the magnetic field. The spin-blockade regions have been
obtained by calculating both the ground and excited states. In double dots the
spin-blockade condition is found to be less stringent than in single dots.
