You can never “just fire” any employee. As an employer, you have a duty to make sure that you follow a fair and legal process when dealing with any employee.
The easiest employees to dismiss are those under two years. If an employee has been working for you for under two years, then they are not protected from unfair dismissal. This doesn’t mean that they have no rights – but it does mean that if they don’t have any protected characteristics (age, race, gender, gender reassignment, pregnancy & maternity, marriage & civil partnership, disability, belief, sexual orientation) then you can dismiss them easily.
If they do have a protected characteristic, then you need to make sure that you jump through all the paperwork “hoops”. You can, for example, fire a pregnant zero hours worker. You will however need full documentation to prove that it is due to performance, and not due to the pregnancy that they are being dismissed.
We would always recommend checking about firing a zero hours or casual worker with an HR professional, mainly because any employee can start a tribunal free of charge. And if they accuse you of discrimination, you will need to prove you are innocent. They will not have to prove you are guilty. The financial award if you lose a tribunal case for discrimination is unlimited (over £2 million in the 2022 case Macken v BNP Paribas).
If a zero hours or casual worker has worked for you for over two years, then you have to follow the same processes as you would for a full time or part time worker. After the two-year mark, all zero hours or casual workers are protected from unfair dismissal. This means that you have to follow a disciplinary process, a restructuring process or do a settlement agreement if you want to part company with them.