You can only be committed against your will if a mental health professional determines you to be an active risk to yourself or others. The key point there is that you have to be evaluated by a mental health professional: and it's not like there are psychologists roaming the streets evaluating strangers. There are only a couple of scenarios that can result in being evaluated: voluntarily (i.e. seeing a therapist) or by some sort of court mandate (if a person accused of a crime displays signs of mental illness, often a forensic psychologist will be asked to evaluate them). Basically getting a stalker committed requires getting them charged with a crime or in some way in front of a judge.

Also keep in mind the bar for committing someone against their will is pretty high. For instance, a person with schizophrenia who is homeless and spends their days talking to voices in their head has a very poor quality of life and is likely to die due to disease, starvation or exposure, but is not suicidal and thus cannot be committed, at least not long term. Sometimes that person can be committed for a few days for evaluation and observation, but no longer.