i agree its irritating sometimes. i was playing a large single star map, the AI had fortified a single GW chokepoint that would hae very effectively blunted just about any attack i could have made... in the end he surrendered to an enemy AI who had been sending single ships to die in that GW for about 2 hours or so...
i dont believe you can turn surrendering off, but the new expansion is about diplomacy etc, so maybe the devs will include something about surrendering...
alternatively, Original Recepie Sins AI dont surrender all that easily
alot of people have said that instead of surrendering, maybe the AI should get a damage/resource boost so it can make one final push (or make the last battle interesting) before kicking the bucket... id go for that... alternatively, if the AI surrenders, make that a way to enable random events, i.e. splinter factions in the enemy empire will attack your fleet or planets randomly... or the surrendered empire wants your help quelling an uprising at one of his planets etc etc