Make parasites melee ships with a slower attack rate (5 seconds or so?) to simulate them being more like boarding action ships, and give them self-damage. They attach to an enemy ship, start sending their boarding teams through, and eventually they're going to run out of guys to send through. Since parasites can only reclaim that which they kill IIRC (rather than tagging ships like Leech starships, correct me if I'm wrong), this would make it far harder to sustain a giant reclamation campaign with parasites.
For Leech starships, suggest they generate resources on attack rather than reclaiming. It has a similar effect - you get money to get more ships, but you still have to build those ships, and those ships have to be things you have already gone through the trouble of unlocking. Maybe even let it reclaim ships that it itself kills (which will happen rarely, but suggest boost to a few higher damage attacks than its current machine gun spam if these changes are made).
If a player REALLY wants to reclaim things, they can always deploy mercenary parasites, which perhaps (because of their cost) shouldn't self-damage.
Since I've already brought it up, quick plug on Mercenary ships - drop costs, or give them some nice boost over normal human ships (example above, mercenary parasites don't self damage, beam frigates maybe have 3x attack, etc.) I never use mercenary ships because they cost so much and die really easily in normal circumstances, and I'd rather not be paying so much for something that's just going to sit around my base.