If solitary confinement should be kept, then it should be adopted in a different way. I sometimes go on meditation retreats, and I pay quite a bit of money to have extended solitary time. This is because I would NOT be spending time proliferating thoughts, but would be learning techniques that can calm my mind etc. Such techniques could be adopted in prisons. The following article (related to detention in schools) is also relevant to this issue: http://www.cbsnews.com/news/meditation-students-mindful-moments-program-robert-w-coleman-elementary-school/
We need to remember that if a person is agitated and angry, the chances are that he would further proliferate anger. This rumination and mental proliferation, etc., can eventually lead to mental health problems (research supports this – even changes in the brain happen under severe stress). So, perhaps a person confined to a solitary cell can be provided with resources that would be helpful for him to calm the mind, etc.