  We consider nanojunctions in the single-electron tunnelling regime which, due
to a high degree of spatial symmetry, have a degenerate many body spectrum. As
a consequence, interference phenomena which cause a current blocking can occur
at specific values of the bias and gate voltage. We present here a general
formalism to give necessary and sufficient conditions for interference blockade
also in the presence of spin polarized leads. As an example we analyze a triple
quantum dot single electron transistor (SET). For a set-up with parallel
polarized leads, we show how to selectively prepare the system in each of the
three states of an excited spin triplet without application of any external
magnetic field.
