Yes. The revelation contract proposed by a genealogist is subject to consumer protection rules (Rep. Min. Pajon au JOAN du 23-10-2000 n° 50427), and consequently to the rules concerning door-to-door sales.
You therefore have the 14-day withdrawal period provided for in Article L.221-18 of the Consumer Code applicable to contracts signed off-site. You can use the standard withdrawal form provided by the genealogist in the contract, without having to justify your decision.