You can offer him a contract as consultant for a time. By hours, just in case that a problem appears.
But I think that the best solution is the preventive one.
Encourage pair programming, or a second developer for every stuff developed.
In some newspapers they use to have peer-review, all the stuff developed for one should be reviewed for another random workmate. You may assign a review partner for every developer.
In general I don't advice to outsource tasks that can not be reviewed and fully understood by an internal developer.
For the people that go away, you can involve the substitute a period before it leaves. The first tasks could be documenting the existent code. If some question arises there is time for direct explanation.
For further actions, you may consider blocking this person and/or reporting abuse
We're a place where coders share, stay up-to-date and grow their careers.
You can offer him a contract as consultant for a time. By hours, just in case that a problem appears.
But I think that the best solution is the preventive one.
Encourage pair programming, or a second developer for every stuff developed.
In some newspapers they use to have
peer-review, all the stuff developed for one should be reviewed for another random workmate. You may assign a review partner for every developer.In general I don't advice to outsource tasks that can not be reviewed and fully understood by an internal developer.
For the people that go away, you can involve the substitute a period before it leaves. The first tasks could be documenting the existent code. If some question arises there is time for direct explanation.