If you are using the domain on af dataset or feature class, then you can't delete it. You'll have to find where you are using it (i think there is a script for that) an change that before you can delete.
The following queries can be used with a 93 sde geodatabases. ---- 1. This sql lists all the domains that have been applied at the feature class level:
SELECT DO.owner domain_Owner, FI.defaultdomainname domain_Name,OC.owner Object_Owner,OC.name Object_Name,FI.fieldname Field_Name FROM sde.gdb_objectclasses OC,sde.gdb_fieldinfo FI, sde.gdb_domains DO WHERE OC.id=FI.classid AND DO.domainname=FI.defaultdomainname AND FI.defaultdomainname IS NOT NULL ORDER BY domain_Owner, domain_Name,Object_Owner,Object_Name
2. This sql lists all the domains that have been applied at the subtype level:
SELECT DM.owner domain_Owner,DM.domainname domain_Name,OC.name Object_Name,ST.subtypename Subtype_Name,AR.fieldname Field_Name FROM sde.gdb_domains DM,sde.gdb_objectclasses OC,sde.gdb_subtypes ST,sde.gdb_attrrules AR,sde.gdb_validrules VR WHERE (AR.domainname=DM.domainname) AND (AR.ruleid=VR.id) AND (VR.classid = OC.id) AND(AR.subtype=ST.subtypecode) ORDER BY domain_Owner,domain_Name,Object_Name,Subtype_Name ============