Here's how to stop users from advancing workflow steps if all actions haven't yet been completed.
Go To Settings > Modules > And select the module you'd like to apply the restriction to.
Click on the workflow LINK you'd like to apply the restriction.
3. Click the [Add Rule Restrictions] button underneath the 'Restrict State Change To' heading.
4. Set the rule to "All Actions Completed" > "Equal" > "True" and click Update.
And you're done! Bare in mind that this applies to ALL actions in the record; not just the actions assigned to the user.