Resource Governor would be a perfect fit for this.
In Standard Edition there's not much you can do. You could refuse connections from an application server in a logon trigger, but that would cause errors in the application and I have no idea whether that is ok for you or not.
However, instead of limiting resources or refusing connections, I think you should try to identify the root cause of the performance issues and fix them. Have you tried that?
-- Gianluca Sartori