Hello Audioipse,
No, there is currently no setting like that. If you add a phone number to your blocked list in Settings > Phone > Blocked Contacts, it will completely block that phone number. There is no controls in there to just have only calls or only messages blocked.
You're right, Do Not Disturb isn't really useful in this situation because it will stop all notifications from everyone, but one thing you can do is just turn off notifications for that one specific message thread. Go into Messages, select the message thread from this number you don't want messages from, select the number or name at the top of the convo, then scroll down and turn on the Hide Alerts option. While you'll still get messages from this person, you'll no longer get alerts when they come in so your phone won't be bombarded with sounds/vibrations and items in Notification Center.
If you would like to see an option added that would allow you to block only messages or only calls, you can make that suggestion to Apple through their feedback site here: Product Feedback
Cheers!