Bay 12 Games Forum

Correct their map to our liking. Aka all of our territory they claimed to be theirs and a generous swathe around it are to be redrawn as our borders to make this perfectly clear.
Afterwards demand their proper borders to be shown, if they want to hide their homeworld they are sadly mistaken.
After they have complied give them a map of our borders.
Should they not comply with any of this, board the vessel and take the crew via non-lethal means. Proceed to keep them as "Guests" and interview them about their species, nation and similiar things.
Prepare an Exploration unit to make sure those territories of ours they have previously claimed are not actually inhabited by them.