To restrict the functionality of a system. If network administrators lock down client desktops, it means that users can perform only certain operations. For example, they might not be able to install new applications. The term is also used to configure a computer or service to prevent unwanted intrusions such as viruses or spam. In such cases, the system may be said to be "locked down."
Lock Down vs. Lock Up
Lock down deals with restrictions, whereas "lock up" refers to not working (see abend