The pasuk in this week's Parhsa states קץ כל בשר בא לפני כי מלא הארץ חמס מפניהם (בראשית ו:יג) "The end of all flesh has come before me for the land is full of theft." Rashi quotes the Gemara in Sanhedrin which says שלא נחתם גזר דינים אלא על הגזל. The irreversible decision of the judgement was only because of theft. The Sefer Be'erat Yitzchak gives a very extensive analysis on the effects of stealing on the thief, his ability to do Teshuva, and why it was גזילה specifically that caused the irreversible decision to bring the flood on the earth. Why, asks the Be'erat Yitzchak, is גזילה considered to be a greater sin than שפיכת דמים and עבודה זרה? After all, the last two obligate the individual to be יהרג ואל יעבור, "be killed rather than transgress," something not required for the prohibition of theft.
Furthermore the Gemara Sanhedrin (דף קח) which Rashi quoted says that the generation of the flood committed all sins, but the final judgement was for theft. The expression נחתם גזר דינם, indicates that their fate was sealed and irreversible.
Why was the decision to bring the flood irreversible? Why was Teshuvah for גזלה not possible? After all, there are many examples of how one can do Teshuvah for almost anything- even the greatest sins. One can achieve eternity in an instant (עבודה זרה יז.).
The following Halacha demonstrates this point: if one says to a woman, you are betrothed to me on condition that I am righteous- even if he is completely wicked, she is betrothed, for the intention of repentance may be in his mind (קדושין נט:). גזלה, however, has certain requirements for a proper Teshuvah. The stolen items must first be returned. A corrective action is required. Whereas, the more severe sins require only the individual's decision to repent, stealing requires more- returning the stolen property.
Returning the stolen items can become complicated, particularly if the stolen item has already been built into someone's house. This difficulty in doing Teshuvah for גזילה was addressed by the Chachamim in the תקנת השבים, the enactment to encourage for Teshuvah, and the תקנת מריש (סוכה לא.) enactment of the built in beam which exempted the thief from returning the stolen good (but required him to pay its monetary value), paving the way for Teshuvah.
We can now easily see why the Gemara says שלא נחתם גזר דינים אלא על הגזל, the final decree was made only because of stealing. It was clear that they would never do Teshuva, for even if they had the intention of doing repentance, they would never have achieved it because the character of the גזלן is not to make ammends by returning the stolen goods. Therefore, the חתימת גזר דין final decree was on חמס.