You are currently unable to automatically exclude users from ticket assignment based upon time off/calendar entries. To prevent these technicians from being assigned, the simplest thing would be to assign to the group initially and then have another user assign to the appropriate technician.
In addition, you could also use Ticket Workflows to make this assignment process easier. You could use an Approval step with an approval mode of Any and set the technician group as the approver. Although the group would be marked as responsible instead of a specific user, any of the technicians in that group could approve the step for the workflow to advance.
If you would still like to automate this process, perhaps you could add a Web Service step that determined if a given user was working that day and handle the response appropriately. However, this type of workflow configuration is more complex. If you would like further explanation of this step or any others, I recommend our Types of Ticketing Workflow Steps knowledge base article.
I hope this helps to explain the limitations with automated ticket assignment and time off entries. In addition, hopefully the ticket workflow configuration above would work for your organization. Please let me know if you would like additional clarification or there's anything else I can do for you.