When I switched jobs earlier this year, I did not allow my new employer to contact my old employer. My old employer was angry with me for leaving and may have overlooked 2 years of good service due to emotional anger. I did, however, proved a full list of reliable references that could vouch for my work ethic, character, etc.
Usually, a question on many employment applications is "May we contact your current employer?" Typically, I'm sure the box "No" is checked.