Can we keep 2 weeks wages as security?

We are a small business just about a year old. We have problems with some staff leaving without our stipulated 2 weeks notice .  Our salaries are not the most lucrative around but they are fair. Staff are fully aware of the salary when they apply and accept the job.

Can we legally include in the contract that we will be holding two weeks wages as security, so that they lose it if they leave without the notice?

It sounds harsh, so are there any alternatives?

Appreciate all the answers, as our accountant is not sure either.

Thanks