the simplest solution that comes to mind would be to do something like:
local stringvar addr:="";
addr:= your exising statment;
addr:=trim(addr); //just to be safe
if right(addr,1) = "," then
addr:= mid(addr,1, len(addr)-1);
this should create the variable that is everything that you are currently displaying, strip off any spaces at the end, and then check if the last character is a comma, if it is, remove it, and finally display the result.