The distinction between किसी and किसी भी is not as marked as 'anybody' and 'somebody'. किसी can mean both 'anybody' and somebody' while किसी भी is like 'just anybody'/'anybody at all'.
When you negate the sentence, both of them किसी से पैसे मत लो। and किसी से भी पैसे मत लो। become 'Don't take money from anybody'/'Take money from nobody'. The sentence with भी is just more insistent.
If you want to say 'Don't take money from somebody' (ie, a specific person), you'll have to use different phrasing, which may involve defining exactly who that somebody is.