Locking up men does nothing more than keep them off the streets. 

字面翻译是：把人关起来只是让他们不上大街而已。

引申意思是：锁住人身，关不住人心。

鉴于网友的实际情况，我觉得不要细分。最好按照固定搭配记忆：

nothing more than = nothing but：仅仅是。

keep 用原形是因为主句有does （do 的任何形式均可）有关。例如：

The boy did nothing but cry all day. =

The boy did nothing more than cry all day.