To protect you and your company from criminal charges, civil litigation, or business loss due to unethical acts of others, conduct a thorough background investigation before you hire employees or consultants, especially if they will have administrative privileges on your systems and network and/or access to sensitive data. This takes extra time, but can prevent a much bigger loss of time, money, and client goodwill (which may be irretrievable) in the long run.
Some other suggestions include
Give consultants only the access that they will actually need to do the work.
Require regular accounting of consultants' activities.
Check billing statements carefully and ensure that work charged for was actually performed. ...