Wow you're being so unimaginative here. There are so many ways. Mandatory contraception. Abstinence contracts. No money if you get another pregnancy. Regular pregnancy tests.
When a person is involved in a major car crash it's reasonable to suspect that they may have been under the influence of eg. alcohol. That's why I asked what portion of welfare recipients in the US were on drugs. I hear about a third of fatal car crashes in the US involved at least one drunk driver. Are we talking similar numbers for welfare recipients on drugs?