“smooth ones.” The context shows that these “smooth ones” were the smooth stones of the ravines that were used as idols or were used as part of the idol worship ceremonies.
“how can I relent?” The Hebrew word translated “relent” is nacham (#05162 נָחַם), and here it refers to God backing off of the consequences that were coming on the people for their sin. In this context nacham could also be translated “change my mind.” God interacts with people and will sometimes relent or change His mind about something if people have a change of heart and action. The NET text note makes a good case for the fact that the last phrase might also possibly be translated, “Because of these things I will seek vengeance.” [For more on God changing His mind or having regret, see commentary on Jer. 18:8].