Plus Ultra
Conquistador
There's also MAD... you can nuke someone's spaceport, if you're prepared to be nuked back. If your opponent can build a spaceport, chances are he's got some pretty advanced nuclear weapons.
There must be some way to counter nukes, like some missile shield system. For bombers and nuclear bombers, the answer probably is the classic interceptor/anti-aircraft gun combination.
Agreed, a combination of MAD, diplomatic penalties and ways to counter nukes are to be expected.