The API doesn't directly support the "Break Link" command, although that is on our backlog to provide in the future. However, depending on how the occurrence was created you may not need it. If you're creating the occurrence in your program, the second argument in the addByInsert method specifies if you want to create a link or not. If this is False, then there is no link and the contents are copied into the current design. Below is some code that demonstrates this.
def breakLink():
ui = None
try:
app = adsk.core.Application.get()
ui = app.userInterface
# Find the file to insert.
fileFound = False
proj = adsk.core.DataProject.cast(None)
for proj in app.data.dataProjects:
if proj.name == 'Junk':
file = adsk.core.DataFile.cast(None)
for file in proj.rootFolder.dataFiles:
if file.name == 'CAMTest':
fileFound = True
# Insert the file into the active design.
des = adsk.fusion.Design.cast(app.activeProduct)
occs = des.rootComponent.occurrences
occ = occs.addByInsert(file, adsk.core.Matrix3D.create(), False)
if fileFound:
break
if not fileFound:
ui.messageBox('The file was not found.')
except:
if ui:
ui.messageBox('Failed:\n{}'.format(traceback.format_exc()))
If the occurrence already exists and you need to break the link then there is a workaround where you can call the "Break Link" command. Because the "Break Link" command doesn't need any input besides the currently selected occurrence, selecting the occurrence and execute the command is all you need. Here's some code that illustrates that.
# Break the link.
ui.activeSelections.clear()
ui.activeSelections.add(occ)
breakCmd = ui.commandDefinitions.itemById('BreakXRefLinkCmd')
breakCmd.execute()