The Torah in Parashat Re’eh (Debarim 15:2) forbids claiming a debt after the Shemita year – “Lo Yigos Et Re’ehu” (“One shall not claim his fellow”). The Shemita year features a number of special laws, such as the prohibition against agricultural activity, and it also cancels outstanding debts. Whether a person had lent his fellow money before the Shemita year or during the Shemita year, he is not permitted to claim the debt after the final day of the Shemita year (29 Elul). He may claim the debt at any point during the Shemita year, but not after the Shemita year ends.
A person who claims a debt after the conclusion of the Shemita year has transgressed this prohibition. He is not, however, liable to Malkut, since this prohibition is transgressed verbally, by asking for the money, without performing an action. Although it is possible to violate this prohibition through an action – such as by trying to forcefully seize the money from the borrower – nevertheless, the violator is not liable to Malkut. The Sefer Ha’hinuch maintains that if a Torah prohibition can be transgressed without an action, then it is not punishable by Malkut even if one transgresses it by performing an action. Therefore, even if one tries to forcefully seize a debt after Shemita, he is not liable to Makut.
The Sefer Ha’hinuch writes that this prohibition applies on the level of Torah law only in the times of the Bet Ha’mikdash. Nowadays, this prohibition applies M’de’rabbanan (by force of Rabbinic enactment).