If an employee is guilty of gross misconduct then an employer is likely to be able to fairly dismiss them even if they haven't previously received any warnings. The conduct must be serious enough to be considered gross misconduct and you should ideally have a Disciplinary Policy which contains clear examples of this.
It's advisable for the employer to undertake a disciplinary process before deciding to dismiss an employee which will include carrying out a reasonable investigation prior to making a decision, otherwise any dismissal might be unfair and potentially in breach of contract.