Suicide is already effectively illegal. Many easy, painless methods are outlawed or not given to the mentally ill (drugs, guns, assisted suicide). Suicide-attempters are force-treated. There is tremendous social pressure to not commit suicide, including inflicted guilt. (For detailed arguments, just read Sister Y's linked blog.)

Even if suicide is discouraged, no-one is likely to compel unhappy people to extend their lives. Most people think life extension is immoral; they won't object to anyone turning it down.