I have found on the web a useful command to delete .svn folders in your local svn repository. I found it here.
The command line is as follow :
find ./ -name .svn -exec rm -rf {} \;
The command find will search in the ./ directory each element whose -name is .svn. Don’t forget to go into your svn directory with the cd command. After each successful research, the -exec command will execute rm -rf with the path found by find put into the {} brackets. The result is that each .svn folder found is deleted.