Is it permissible to refuse the husband who engages in multiple relationships with women, and how can he be prevented from being alone with female colleagues and riding in the car with them?

You are not permitted to withhold yourself from your husband because of his relationships; rather, that might lead him to engage in more of them. You should advise him and remind him of Allah regarding the impermissibility of corresponding with girls, and of unlawful seclusion and mixing. Inform him of the obligation to leave his job if it will lead him to those forbidden acts, and that he must search for a job that does not involve anything unlawful. And supplicate to Allah for his guidance.