An employee has been accused of attempting bribery abroad, and now we are banned from the country in which he was operating. What can we do?

The use of bribery abroad is an offence under UK law, so you have presumably made it plain to your employees that you discourage it. Your first step should be to set up an investigation. If the investigation suggests that the charge is justified, you ought to refer the matter to the police, regardless of any disciplinary action that you may take against the employee.