In the circumstances explained, it's implied that you are also willing to join Company A in due course. This gives the impression that the job offer from Company B may not be your preferred choice. Additionally, it's possible to inform Company A that they may halt further processing of your joining formalities. You are free to write a convincing email expressing your current unavailability while leaving the option open for the future. However, based on my knowledge, many employers may not be inclined to consider a candidate who has previously turned down their offer unless the candidate shows exceptional promise and potential that aligns well with their company. Many employers prefer to engage in 'headhunting' from their competitors, especially when they are eager to acquire new businesses or clients.