Part of the answer to at least preventing spread is to provide clean fresh water to avoid them having to drink from a source that is contaminated.
The path probably isn't part of the problem, Ark-Klens is still the best cleanser disinfectant for the bird table etc. Ark-Klens will kill Trichomonas. If you are concerned about other organisms such as bacteria and viruses I would suggest either Tamodine E (which may stain the gravel so test it first), or Virkon Aquatic (the Aquatic form has no colourant or scent, so nothing to put off wild birds).
Vetark have recently launched CitroSan which is an extract of oranges ie. a natural product with biocidal properties to prevent the transmission of Trichomonas in the bird bath.