This will work in SQL 2005...
Create trigger trgHmm1 on DATABASE for CREATE_TABLE, DROP_TABLE, ALTER_TABLE
AS
ROLLBACK;
GO
Create trigger trgHmm2 on DATABASE for CREATE_TRIGGER, DROP_TRIGGER
AS
ROLLBACK;
GO
not sure about this one though:
Create trigger trgHmm3 on DATABASE for CREATE_TRIGGER, DROP_TRIGGER, DISABLE_TRIGGER
AS
ROLLBACK;
GO
Kinda makes a strong case for really keeping tight control over who has db_ddladmin in a database doesn't it?