I don't think you're down and out quite that easily. One planet lost, out of 10 or 15, on a 30+ planet map isn't that big a deal. One planet out of 5, on a 10 planet map ? Thats a different story. While you've lost a fleet, you still have an empire and it's resources, given some time, you can recover. So, you'd have to play for time, and a lot depends on your opponent in this case. Even if they drove thier fleet directly to your home world, it would still take 4 or 5 minutes to make all those hops..
If they are totally warlike and haven't invested in economy and infrastructure, the loss of your fleet is a momentary irritation. If you're out producing them ( or have the capability ), then you could have 5, 6, or even 15 ship yards cranking out a replacement fleet. By the time the enemy had secured the planet you were both fighting over, your replacement fleet would be ready to counter attack. It would be a grinder, but you could eventually win by attrition.
But, even assuming you both have comparable economies and empires, there is still a lot you can do. Re-enforcing the next choke point with some static D, a few repair bays, and new ships as they roll off the line helps. This would offset some of the numerical advantage. You would also know his fleet composition and could build perfect the counter-fleet so the new fleet would be ( hopefully ) much more effective 'next time' .
You could go with the typical Uber-weapon, and that will work if you have more then one. But even with a Novalith, it takes two working together to nuke a maxed terran planet. If you didn't already have them researched and built, or at least under construction, you'd be wasting over 8k cash each ( plus reasearch ).. that would buy a hell of a fleet ( 50 HCs or 90+ LRMs ? ). I've never seen this work as a game changer in a player vs player game. You'd need a minimum of three to be have enough fire-power to cripple his economy. One shot would just cause them to launch a full-on suicidal attack at a time when your own fleet couldn't respond. A single novalith might get a second shot, but probably not a third, IMO. BTW, if you did have three sitting around, and equal fleets, I doubt you'd be in danger of loosing !
But, I think culture is the best option. Culture buys you lots of time, which is what you need in this scenario. They could blast your planets, but not colonize them. All you'd have to do to re-take them is run a colony frigate by. They would also be several jumps away from repair bays and re-inforcements. Eventually the difference in the length of the supply chains would allow you to play an attrition type game and whittle him down. One reason every planet I take gets a broadcast center for over-lapping coverage.