因foreign key 带有级联delete 或update无法创建触发器

触发器如下：目的是禁止删除表中内容，级联字段一定要，不能去掉，有办法改进吗
CREATE TRIGGER candel来款
ON dbo.来款
instead of DELETE
AS
declare @bh as char(20)
if exists(select * from deleted where  来款 is not null or 客户id is not null)
begin
raiserror ('非法操作，请联系管理员！',16,1)
rollback tran
end
elsebegin
select @bh= 客户id  from deleted
if  exists(select *  from 来款 where 客户id=@bh)
delete  from 来款 where  客户id=@bh
end