Oooh, this is by far my favorite, that's why I saved it for last. If you're to the point where you're seriously considering disabling solitaire, this reveals a number of things about the organization:
1) The I.T. staff and/or managers are unapologetic control freaks and perhaps even proud of it.
2) You don't trust your employees to actually be productive on their own.
3) Your hiring standards are probably pretty low.
4) You have unrealistic expectations of employee efficiency.
5) Morale must really be in the toilet already.
6) It's solitaire for fuck's sake, possibly the most boring game ever devised. If your employees are playing it instead of whatever they should be doing, that means they have no motivation to work, which means management should be the ones to get their lunchtime games taken away, not the employees.
So what if it was an application that carried malware that you were blocking? Solitaire is a particularly silly example, but is representative of any application. It doesn't take much imagination to come up with an application that you might legitimately want to block. But you seem to be having such a good time, don't let me stop you...