This is an interesting and (somewhat cyclical) caveat in the GNU GPL, both version 2 and version 3. There is an accepted answer to this question, but I'm on a bit of a personal mission to help clear up a lot of the repeat confusion regarding software licenses that I see posted here.
The direct answer to your question is, you can do whatever you want with anything you own. The GPL is a license that you chose, in order to assert your copyright and ensure that everyone who distributes your software gives recipients the same freedoms you had with it. The GPL assumes, however that the project will be "GPL Forever", and doesn't really consider your scenario.
The teeth of the GPL kick in only when you distribute software1, and yes it does guarantee access to source code. However, in your case, you are perfectly free to take the project down and no longer provide source code. If you feel really guilty about it, you can take yourself to court - which is really the point of the matter. The license protects your wishes by asserting your copyright. There is no recourse to speak of if you decide to make the code proprietary because you own it.
This is often not the case in projects that have many contributors who don't sign copyright assignments prior to contributing. If that were the case, you would no longer be the sole owner of the work. Then things would get interesting:
- At this point, you can't remove the project without permission from all copyright holders
- At this point, you need permission from all copyright holders to create a proprietary fork, or a dual license scheme.
The thing to do, in either case is to make a private / proprietary fork of the project and declare it dead on the SF project page. As you have seen, nothing prevents someone else from publishing your code base, up to the last GPL covered revision and going their own way with it. They are simply exercising the rights you gave them at the time that they received the software. That is just the GPL working as advertised, ensuring whatever you released under it remains free - no matter what :)
As far as the license goes, no, you didn't make a technical mistake. However, you did make a bit of a social blunder.
1 The same teeth also ensure that there is no discrimination against fields of endeavor (e.g. run the program, for any purpose) or any other additional restrictions beyond the scope of the license, however that's sort of irrelevant for this question.