Would you be able to create an extra-dimensional space large enough to trap them once they cross the portal? What I mean is have an illusion up so when they cross through, they walk right into an extra-dimensional space. Once inside, we destroy it.
(It's impossible for Russel to hide the fact that he is trying not to be condescending)
Extradimensional spaces are typically bags of holding and portable holes and not useful for trapping giants, or really anything larger than a pixie. Spells which create an extradimensional space, such as a rope trick, can only be entered voluntarily, which means they are almost equally useless. We could try to set up a Gate immediately adjacent to the portal so that those passing from Cald immediately passed to another plane, however there are problems with this plan as well.
First, we have no reason to think whoever comes to activate the portal will come through the portal. I would expect them to already be here and thus we could find ourselves in a position of having to defeat them before they could open the portal and thus have wasted all the time until then.
Second, assuming they do come from the otherside, our enemies are in the nature of a military and therefore they will most likely send a reconnisance first. We will then be forced to choose between destroying that element and loosing our chance to strike at the leaders; or waiting for the main force and risk finding ourselves overmatched. Luigi and I recently participated in a true large scale battle and I for one have no illusions about our ability to defeat a sizeable force of giants.
My vote remains be done with it now if we can, either by disjunction or Gate.
