... clear the drop-down lists in TortoiseSVN?

As of Version 1.4.0 and higher, you can clear all the stored data from the settings dialog of TortoiseSVN. Just click on the corresponding button.

For older Versions of TortoiseSVN, you have to delete the registry keys manually:
Delete the registry keys:

  • Commit dialog per-repository comment history
    HKCU\Software\TortoiseSVN\History\commit*
  • Repo browser per-repository URL history
    HKCU\Software\TortoiseSVN\History\repoURLS
  • Repo browser general URL history
    HKCU\Software\TortoiseSVN\TortoiseProc\repoURLS
  • Commit dialog comment history (older TSVN versions)
    HKCU\Software\TortoiseSVN\TortoiseProc\commit