Can we legally prevent a sales team member to take clients when he/she leaves our startup?

You should insert a non-compete clause in your employment contract - for a specified period of time, after an employee leaves your company, they cannot engage in the same/competitive company, serve the same clients, whatever - it's pretty much standard employment contract law.

I would advice contacting a French lawyer on the topic, but as far as I know French law allows such clauses, provided there is a financial compensation.

I.e. the contract may indicate something like "the company A reserves the right to prevent Mr. X from working with competitors or company's customers for Y months, should the company A decide to exercise this right, a compensation Z will be paid to Mr. X".

As I remember, without the explicit compensation, this clause will not be valid in France. Also, the compensation must be a certain percentage of the salary, i.e. 1€/month won't do.

This is what I remember from my time in France, a local lawyer will certainly advice you more precisely.