If I recall correctly, I remember someone saying in a shiur a long time ago that a dishonest person cannot do teshuvah. Is there any source for this claim?
Anyone can do Teshuva but a dishonest person can’t always pay back and make amends.
The Gemora (Bova Basra 88B) says קשה עונשן של מדות יותר מעונשן של עריות
The punishment for using false measures is more severe than the punishment for transgressing the prohibition of forbidden sexual relations.
The Gemora goes on to discuss why and says מאי עודפייהו דהתם אפשר בתשובה והכא לא אפשר בתשובה
in the case of one who engages in forbidden relations, he has the possibility of repentance. But in the case of one who uses false measures, there is no possibility of repentance because he has no way of knowing whom he cheated, and is therefore unable to return the stolen money.
However the Gemora (Bava Kama 94B-95A)discusses Teshuva for theft. Even if the thief doesn't know who he stole from he can try to pay back by paying communal needs. (Tosfos says that doing so isn't a real repayment)
It is said over in the name of the Chofetz Chaim that Gezel Shinah (keeping someone else up) has no way to be paid back