Can Solemn Judement be used to stop Sangans' effect when he is sent to the graveyard?

nope

solemn only negates summons, spells and traps

not monster effects

If you solemn sangan's summoning he will not get his effect.

It isnt that Solemn will stop the effect...it is you are negating his summon which entails that he is not normal summoned...So since he was not summoned successfully and it was negated, it is like he never hit the field so he is simply destroyed not sent from the field to the graveyard. Which will not activate his effect.