No they can't - it's illegal
Breach of contract by an employee
If you breach your contract, your employer should try to settle the matter with you informally, but they can sue you for damages in the same way you can sue them.
Damages are only awarded for financial loss (for example, if you don't give enough notice, damages might be for the extra cost of hiring temporary staff to do your work, or for lost revenue. You would still have the right to wages you earned before you left, plus pay for untaken statutory holiday).
The most common breaches of contract by an employee are when you quit without giving (or working) proper notice, or when you go to work for a competitor when your contract doesn't allow it.
http://www.direct.gov.uk/en/Employment/Employe es/EmploymentContractsAndConditions/DG_1002752 1
As I said, if they want to recover monies they have to sue the employee