Say you had an anti-neutron and a proton. If I remember right, a proton has two up quarks and 1 down quark, and an anti-neutron would have 2 anti-downs and 1 anti-up. Assuming the individual quarks annihilated, that would leave what? An up quark and anti-down quark? I suppose it seems possible, and charge is conserved, so there's no problem there.
But as far as I know, quarks have never been observed to exist individually, so it would really suprise me.
I don't know. Hopefully someone who knows something will help us out here.