Is abstaining from food and drink after noon with the intention of disciplining oneself for having committed many sins, without the intention of fasting, considered an innovation (bid'ah) or permissible, and how can one go to great lengths in repentance to avoid repeating sins?

It is permissible to discipline the soul by obligating it to certain acts of obedience, and likewise by depriving it of certain permissible things, such as abstaining from food for a period that does not harm it. This is permissible and Islamically مطلوب (desirable). The Prophet, peace and blessings be upon him, and his companions would go hungry and reduce their consumption of desired foods. Al-Khadimi Al-Hanafi said that making the soul hungry—as long as it does not incapacitate it from acts of worship—is recommended so that it may gain strength for obedience. If it commits a sin, one should punish it with hunger or by refraining from looking at forbidden things, provided that this is done within limits that do not harm the soul or incapacitate it from performing its obligatory duties. If it reaches the point of harm or incapacitation from obligatory duties, its doer becomes disobedient.