When sending alerts for an entire Trello board to the specified Slack #channel, there is currently no way to limit said notifications to a single list within that Slack board.
The only "workaround" is to create an alert for a single Trello Card rather than the entire board. Assuming the Card has only 1 list the Slack channel will only get updates for that single list.
To do this, open the specific Card in Trello and with the Slack power-up enabled select "Send To Channel" or "Send via DM".
If your usecase is, for example, a Slack board where some users Care about the entire board's updates while others only care about updates made to a specific list or two, I would separate those users into 2 channels and use the above workaround.