To clarify a point made earlier, the NCAA does indeed manage the National Letter of Intent program. It used to be administered out of the SEC offices, but that responsibility was shifted to the NCAA a few years ago.
And, as SDBB copied from the NLI website, there is a recruiting ban after a prospect signs an NLI. That is the primary reason that the NLI was created in the first place, was to put a stop to prospects continuing to be recruited after they had signed with a school. Coaches and athletic administrators (who after all, came up with the idea) wanted to "lock in" a prospect and not worry about them being continually pursued by other programs (and the signing schools didn't want to be pushed to continue their recruiting efforts on those already signed).
Also, while I DON'T RECOMMEND IT, there is another option if the athlete wanted to have some "payback" on this school. The athlete could enroll there for the Fall semester, get their scholarship, cause the school to have to count their scholarship against their limit and their 35 man roster, and then transfer to another Div. I after the first semester. The athlete would have to sit out one year from competition at the next school, but could work out and practice at the school during that year of sitting out, and be eligible to play when Spring 2015 rolls around with all eligibility remaining after having a year to get bigger and stronger.
Finally, to reiterate another point in SDBB's copy and paste of the NLI rules, if the athlete does go the JUCO route to start with, he'll still need a release from the NLI if he plans to transfer to a Div. I (or II) school before graduating from the JUCO. Otherwise, this NLI is still binding on him (assuming the Big 12 school he signed with would have an interest or need for him at that point).