In DB2, how can I disable NLJOIN? I want to indicate to the optimizer, via SET or optimization profiles, that NLJOIN shouldn't be used at all when optimizing a query.

I've been looking for this for a while without any success. I looked for a property that could be set through the db2set command, but the only one referring NLJ is DB2_INLIST_TO_NLJN.

The closest I've found is through the usage of optimization profiles but those are very specific and what I need is a general setting that would apply to any statement being optimized.
You might wish to check out this article on join methods and how DB2 chooses which one when.

Based on this documentation (which is for DB2 8 and I'm assuming it is similar for 9+, though not necessarily), nested loop joins are always available to the optimizer to use. DB2 can select merge sort and hash as the optimizer level is upped from 1 and other things are present (read the article to see which).