@Greymane:

I am afraid the answer is not a clear-cut black and white; it's gray. You see, the skill notes that if you fail by 5 or more while attempting to improve a hostile computer, it locks you out for 24 hours (among other things). This is the only mentioning in the entire section of it taking a direct action to prevent you from trying again. There is, however, and indirect case. If the failure occurs while the computer is no better than friendly by 5 or more, it moves a step down; if in doing so it becomes indifferent or unfriendly, it contacts the administrator about the attempted access. In this case it becomes a roleplay situation: if the player(s) can somehow fool either the computer or the administrator into believing the access attempt was supposed to be authorized (such as if they had already persuaded/tricked the admin in charge or spoofed the login to be someone else who should be authorized), the administrator may tell the computer to override and revert to good standing; should it happen twice in a row (or if the players did not attempt to make their access look legitimate), the admin would likely instruct the computer to lock out all access until they arrive and address the scene (and maybe to contact security). In other words, it becomes a scenario for the players to try to overcome unless the GM decides one way or the other.

While this isn't a direct solution, you could try bumping up the computers' will saves, lowering the initial attitude for the player, both, etc. to try and make the challenges more challenging for that player; there is no rule that states you cannot adjust an encounter (a lot of people seem to feel the storm troopers go down to quickly even at level one and therefore use clone trooper stats instead to bump up the challenge a bit). If said player then gets mad about it, you can pull them aside and tell them that you just wanted to give them a challenge or deal with the situation in the manner you think is best (I do not personally know the people you are playing with, so your results and how to deal with them may vary).

Unfortunately, the only other items in the SAGA rulebook that could be seen as relative to the situation are at the start of the chapter under Trying Again and a single line in the organic variant, Persuasion: Change Attitude. The former states "in general, you can try a skill check again if you fail, and you can keep trying indefinitely. Many skills, however, have natural consequences for failing that must be accounted for." The latter states at the very end that "You may attempt to change the attitude of a given creature once per encounter." You could Rule Ø that the same applies to Use Computer, but that may or may not go over well with your players. One key thing, however, that may be of use to you is that each use of Use Computer: Improve Access takes a full-round action. Therefore (unless he optimized the heck out of improve access), you could add an element of being discovered; a routine check in by guards, cameras getting cycled between channels (or, for even more hacker fun, between frequencies), patrols sweeping sectors, etc. If he takes too long trying to improve his access, someone may notice their presence in the location and proceed to investigate.

I know all the stuff I've said has been RP solutions instead of concrete rulings from the books, but as far as I am aware, there is no official rule that prevents them from doing what they did. Still, I hope something among all this gray was of use to you.