They can't know if there are any Vasudans passing through the node at the moment of detonation for one thing.
The other thing is, would it really be worth the cost? If the blockade is ships only, they could just crash jump away and be back a few minutes later.
And even if they wiped out the whole blockade force, what would it really gain them? Losing those ships won't diminish the GTVAs offensive forces in Sol and might make them fight harder and get more public support, maybe even bring the Vasudans into the conflict full force.
Their supply lines might be a bit more vulnerable for a few days, till the GTVA can get new defences in place, but that's pretty mucht it.
I think the UEF has more to lose by such an action than they to gain.