If you look at any mussar sefer the answer is no, as chazal say אפילו חרב חדה מונחת על ראשו אל ימנע עצמו מן הרחמים. Yet, the Rambam in Hilchos Teshuva (6:3) states explicitly that it could be too late:
ואפשר שיחטא האדם חטא גדול או חטאים הרבה, עד שייתן הדין לפני דיין האמת שיהיה הפירעון מזה החוטא על חטאים אלו שעשה ברצונו ומדעתו, שמונעין ממנו התשובה ואין מניחין לו רשות לשוב מרשעו, כדי שימות ויאבד בחטאים שעשה.
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לפיכך כתוב בתורה "ואני, אחזק את לב פרעה" (ראה שמות ד,כא; שמות יד,ד): לפי שחטא מעצמו תחילה והרע לישראל הגרים בארצו, שנאמר "הבה נתחכמה, לו" (שמות א,י), נתן הדין למנוע ממנו התשובה, עד שנפרעין ממנו; לפיכך חיזק הקדוש ברוך הוא את ליבו.
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וכן ישראל בימי אלייהו לפי שהרבו לפשוע, מנע מאותן המרבים תשובה, שנאמר "ואתה הסיבות את ליבם, אחורנית" (מלכים א יח,לז), כלומר מנעת מהן התשובה.
It is possible that a person can do a great sin or many sins until the judgement for his sins that he did willingly is that he is prevented from doing teshuva and he is not given permission to repent so that he will die with the sins that he did
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therefore the Torah writes that I will harden Paroah's heart ...
and also the Jews at the time of Eliyahu because they sinned so greatly were prevented form doing teshuva ...
We see black on white that the Rambam writes that a person can be prevented from doing teshuva, in other words it is too late.
What do the mussar sefarim do with this Rambam? R' Schwadron, who edited the Lev Eliyahu, asks this question on the Lev Eliyahu in Parshas Bo in a footnote and answers as follows.
The Rambam's proof from Bnei Yisrael in the time of Eliyahu is very difficult, after all the story there ends with Bnei Yisrael screaming ה' הוא האלוקים, in other words doing teshuva. He answers that even if teshuva is closed off tefilla never is. A person can always daven to hashem that the punishment should be lifted and he should be allowed to do teshuva. This is exactly what Eliyahu Hanavi did.
How does this work? When a person is prevented from doing teshuva it means that his yetzer hara is given more power and there is no way for him to overcome the yetzer. However, he can daven to remove this. This is like a person who loves to talk loshon hara. The person knows it is assur but just can't help it. The person may even daven to hashem to help him overcome. This is the same idea here. The yetzer hara is too strong to do teshuva but tefilla can help weaken the yetzer hara and then he will be able to do teshuva.
Therefore, the Rambam is right that sometimes it is too late to do teshuva, but even so the mussar sefarim are right that it is never too late, a person can always ask for rachamim and the ability to do teshuva.
We should keep this in mind in Chodesh Elul that tefilla is never too late.
