What prevents someone from turning the same idea into multiple papers?

Submitting (almost) the same paper, the same content, to two different journals at the same time is bad. Once it comes out, the author will be in a lot of trouble, e.g. reputation, both papers withdrawn, etc.

Reusing old results for a new paper is plagiarism if there are not enough new results. Even if the author of the new paper also wrote the old one, it is still bad. There are cases where different papers by the same author look really similar, for example if the author presents in paper (1) a new technique and then publishes paper (2) on nice results obtained with said technique, he might copy some of the explanations on how the technique works. If this is good or bad seems to be opinion based, all I can say is that it happens.

Regarding your question about what prevents people from doing such things:

• Yes, a bad reputation might be one possible reason.
• Actually, there are people doing this, e.g. trying to submit their paper to different journals at the same time, hoping that at least one of them will accept it. There was a question here a few weeks back from one who got busted doing it and asked for what to do now to save the sinking ship that was his academic career. The reason why it seems that this is not done might be thanks to journals taking measures against it, e.g. not publishing such papers or withdrawing them later on.
• One last point to mention here might be colleagues or advisors. If you feel like it might be a good idea to just spam your results to get them accepted or to get a high paper count, you might get the advice that this is not a good idea in the long run from one of the above - if you are lucky you get it before you do something irreversible.

There may be policies from the publishers which could result in a paper being withdrawn. For example, from Elsevier's policies and ethics page:

Multiple, redundant or concurrent publication: An author should not in general publish manuscripts describing essentially the same research in more than one journal or primary publication.

Anecdotally it's not uncommon for research groups to find ways of getting many papers out of work (Core work, work applied to situation A, work applied to similar but distinctly different situation B...) which is sometimes completely appropriate, sometimes less easy to justify.