In Setup > Project Preferences > Tasks & Issues you can set the default duration type to Simple. But unless you're creating project templates where all the tasks have a constraint of Fixed Dates, I don't believe there's a way to set a default constraint.
Hi Howard - Yeah, to get fixed dates, you just need your Project Owners to adjust both the start and end dates of the tasks. We have a "one-off" project for resourcing, and as long as you touched both those fields, the task constraint changes.