It seems to me that G-d actually DID give the people a chance to repent.
Firstly, it took a year for Noah to build the ark. This was a BIG boat.
People definitely noticed what he was doing. I'm sure that they inquired
what he was up to and were told about the flood that was coming from G-d.
One year - It's plenty of time to get it together.
Secondly, just before the flood Methusalah (sp?) died, and the Chumash
tells us that the people mourned (sat shiva). He was the oldest person who
ever lived, and if you count back, he actually lived in the time of Adam
(at the end of Adam's days). He was totally connected to creation itself.
His greatness was recognized because of this, and it says that when a
righteous person dies, it is a message to the people to do teshuvah
("return"-- often mistranslated as "repent").
So, at the end of the year of building, when the people hadn't got the
message, G-d has a great man die to try and inspire an awakening of spirit
and change of deeds. But, in this case, it still didn't happen and the
flood waters came.