If your probe survives either of the following, you could choose:

1) Hydrogen plasma

2) Nitrosulfuric acid

3) Oxygen at >700K

Dear A.T. Nguyen,

You can try nitrosulfuric acid

Dear Jurgen and Fida,

Thanks for your recommendation.

Dear Jurgen and Fida,

do you have any paper showing the procedure of using nitrosulfuric acid to clean the diamond-coated probe?

For a pure diamond sample, a treatment at 250°C in conc. nitrosulfuric acid is standard.

However, please check whether your actual probe is able to handle that. If it's e.g. diamond-coated tungsten, you shouldn't do that because it will also dissolve the tungsten, completely so you would end up with a coating without a substrat underneath.