Supes purposefully holds back.
It's what ended up getting him killed dealing with Doomsday, and tends to be the reason he runs into initial trouble with some of the villain powerhouses.
The hulk is generally speaking infinitely strong, limited only by his degree of anger. Though, reclaiming Banner's intelligence has proven itself to be an actual restriction to his strength and power on numerous occasions.
Generally speaking, it's not a matter of Supes not letting him get to that point, Hulk would keep getting more and more angry as the fight progressed. Similarly, Supes would start holding back less and less. But by the time Clark realized he needed to stop holding back to deal with Hulk, he wouldn't be able to really handle him conventionally and would have to employ some of the same tactics he used fighting Doomsday at the end.
And lobotomizing Hulk would actually be the absolute worst thing he could possibly do since it's been shown several times that turning Banner "off" removes any restraint Hulk exercises.
Thor learned that the hard way
https://www.youtube.com/watch?v=eN02EcS1WF4So Did Onslaught when Bruce had Jean Gray turn him "off" so Hulk could fight at his full power unrestricted.
The only thing more dangerous than hulk with no Banner to reign him in, is when Hulk and Banner are in agreement (World War Hulk).