How to restrict the level of privileges of applications on jailbroken devices? This question is related to this other question.

After you have jailbroken your device, you have root access to your device and so do apps that you install. 

Can you change the account that an application runs with after you have installed it? This would allow to use an account with less privileges.

If it's possible, then how do you grant/revoke access to data (address book, emails, settings) and APIs (install keyboard hooks, issue network request)?
Someone would have to write a sandbox for jailbroken programs, which would be extremely ironic, because the point of jailbreaking was to escape the sandbox. Normal apps still run under the mobile user when you're jailbroken, though, so there's nothing to worry about there. All jailbreak packages have full access to anything, and there's currently nothing you can do about that. Just play it safe, and only install trusted packages from reputable developers/repositories. A related question that I answered on the same subject can be found here.