I have had several unkillable spids, usually they have been with extended stored procedure calls, SQL can kill itself but can't necessarily kill the xp, this is really a bear...

I have used log shipping, transactional replication, and merge replication. To create several types of report servers and backup servers, with 30+ databases. Log shipping is the best for report...

Usually when I have that problem, is when something is using that linked server, like replication or a job has an open connection to it.