It is a positive commandment to learn Torah. But the concept of bitul Torah seems to imply that it is also a negative commandment not to learn Torah. Where does this concept of bitul Torah come from?
I don't know of any negative commandment per se, and as @DoubleAA pointed out in the comments, the term bitul does not imply that there is one. As for where the concept comes from, here's one place:
Note what meditating on the Law is contrasted with.
The Gemara in Avodah Zarah (18b) citing the Tosefta (ibid. 2:2 in the Vilna Shas edition) also makes this connection; equating bitul Torah with sitting in the company of mockers.
the verse "ki dvar H-shem baza hikaret tikaret" ("For they have denigrated the word of G-d" (Parsha shlach) which the talmud says refers to bitul torah