In my CLAUDE.md I put:

> CRITICAL: Do NOT spawn sub-agents for any reason. Perform all work in the main session. If a task is too large, ask me to break it down manually.

> This is a big task, and can easily get too large. However, sub-agents make the situation worse, and eat through our token budget way too fast. Do not use them.

> Take on manageable tasks. Don't try to do everything at once. When you start on a big task, break it down into smaller tasks, and make sure you finish each task before starting on the next one.

Or actually Claude put it there for me. Maybe it's a bit much, but it seems to work.

I think there's some setting to restrict the number of them, or maybe turn them off. Doesn't happen for me ~ever and it's not my $$ (work) so I haven't really looked at it much.

Such is the nature of tool use