Look for broken or disconnected hose as Jim suggested hoping that is the problem. Since you are under the hood, you may as well check the condition of the vacuum shutoff which is the round thing attached to the back of the fuel injection pump about an inch from the oil filter housing. This round thing has a vacuum hose attached to it. If that hose is disconnected, you will have a hard time trying to stop the engine manually. Disconnect that hose and look inside the hose to see if it is oily. If it is, replace that round thing (easy job but a mistake can blow the engine).
There are multiple reasons for engine not stopping on older car like yours. Cars with vacuum leak or defective vacuum shutoff may still shut off the engine. But the combination of vacuum leak, defective vacuum shutoff, and a weak vacuum engine pump will give you the shutoff problem.
Be patient and check out each area, one at a time. You do need a hand vacuum pump with a gauge to perform tests.