A domain cannot be deleted if it is associated with a feature class or table. Use the Remove Domain From Field tool to remove the association between a feature class or table and a domain.
You can also manage domains in Domains view which can be opened by clicking the Domains button found in the Design group on the Data ribbon.
This stand-alone script uses the DeleteDomain function as part of a workflow to edit a domain.
# Name: RemoveDomainsExample.py
# Purpose: Update an attribute domain to constrain valid pipe material values
# Import system modules
import arcpy
import os
# Set the workspace (to avoid having to type in the full path to the data every time)
arcpy.env.workspace = "data"
# set local parameters
inFeatures = "Montgomery.gdb/Water/DistribMains"
inField = "MATERIAL"
dWorkspace = "Montgomery.gdb"
domName = "Material"
codedValue = "ACP: Asbestos concrete"
codeField = "TYPE"
descField = "DESRIPT"
# Process: Remove the constraint from the material field
arcpy.management.RemoveDomainFromField(inFeatures, inField)
# Edit the domain values
# Process: Remove a coded value from the domain
arcpy.management.DeleteCodedValueFromDomain(dWorkspace, domName, codedValue)
# Process: Create a table from the domain to edit it with standard editing tools
arcpy.management.DomainToTable(dWorkspace, domname,
os.path.join(dWorkspace, domname), codeField,
descField)
# Process: Delete the domain
arcpy.management.DeleteDomain(dWorkspace, domName)
# Edit the domain table outside of geoprocessing
# and then bring the domain back in with the TableToDomain process