Personally, I would tell the person "Go ahead and go now, and we'll just continue to pay your salary to the end of the contract". If the person is really unhappy with the place, having them around bringing everyone down is going to cost a lot more than paying someone 2 months to not be there IMO.
How do you contract this sort of thing? If I wanted to quit and I had a contract like this, I would just stop coming to work, or I would come to work and do a bad job.
Since it's a legal contract violating (by leaving) it could lead to lawsuits.
It's something similar in Sweden, though the contract goes both ways. They can't give less notice for firing you than you can for quitting, so it just damps the system somewhat.
Although it's in the contract, you are always free to re-negotiate a contract, and if both sides agree, then you have a new contract