How can doctors relieve lockjaw?

Tetanus. Toxin produced by clostridium tetani causes a spastic paralysis of muscles resulting in what is known as "lockjaw", among other involved areas of the body. Treatment is use of tetanus antitoxin to remove active toxin from the bloodstream, and support of the patient until the toxin bound to the nervous system wears off, which can take a lengthy time period.
Depends on the cause. Lock jaw or the inability to open or close the lower jaw can be caused by a number of things. In order to treat it, your doctor needs to identify the cause first.