Well, that's how I read it on the Gov. website. How they achieve this I don't know.
"If you’ve committed or attempted fraud, one or more of the following may also happen:
you’ll be told to pay back the overpaid money
you may be taken to court or asked to pay a penalty (between £350 and £5,000)
your benefits may be reduced or stopped
Your benefits can be reduced or stopped for up to 3 years if you’re convicted of benefit fraud. The amount of time they’re stopped for depends on how many times you’ve committed fraud.
Only certain benefits can be reduced or stopped. These are called ‘sanctionable benefits’. But if you commit fraud on a benefit that cannot be reduced or stopped, your other benefits can be reduced instead."
Seems clear. If the fraud is obvious they will take action, but they can't take the money you may need to live on.