Ok, i found out the solution:
alter TRIGGER [trig_deny_ddl]
ON database
for DDL_TABLE_EVENTS, DDL_VIEW_EVENTS, DDL_FUNCTION_EVENTS, DDL_PROCEDURE_EVENTS
AS
declare @command nvarchar(max)
SELECT @command = EVENTDATA().value('(/EVENT_INSTANCE/TSQLCommand/CommandText)[1]','nvarchar(max)')
if @command not like 'alter table%check constraint %'
begin
print 'Strukturelle Änderungen auf dieser DB verboten. Bitte wenden Sie sich an den Administrator'
rollback;
end