ת"ש נפש שבנאו לשם חי מותר בהנאה הוסיף בו דימוס אחד לשם מת אסור בהנאה הכא במאי עסקינן דרמא ביה מת
Come and hear! A tomb built for a man still alive, may be used. If, however, one added a single row of stones for a dead person, no [other] use may be made thereof. This deals with a case where the corpse had actually been buried there.
Based on this the Beit Yoseif (Yoreh Deah 364) cites the Hagahot Asheri that one should not sell broken tombstones, lean against them or step on them. However, over the centuries the enemies of the Jews found creative ways to desecrate Jewish cemeteries. We have seen reports from Ger (Poland), Lemberg (Ukraine), Brest (Belarus) and more recently it turned out that the Wenceslas Square of Prague was cobbled with stones stolen from Jewish cemeteries.
Should Jews avoid going to such places, as by walking on these roads they are clearly benefitting from the desecrated tombstones? Alternatively, does the ruling only forbid Jews to sell tombstones, but if gentiles steal them we shouldn't be concerned?