Another possibility would be to query the user-list with an SPQuery too.
That would be much faster than iterating through all users.
You could use the <In>
Tag inside the query to get all users that have not been assigned by a workflow.
For this tag check: http://rmanimaran.wordpress.com/2011/03/11/new-in-sharepoint-2010-caml-query/
An example on how to exclude items from the page:
<Where>
<And>
<Eq>
<FieldRef Name="Specialization"/>
<Value Type="Lookup">SQL Server</Value>
</Eq>
<NotIncludes>
<FieldRef Name='Specialization'/>
<Value Type='Lookup'>Crystal Report</Value>
</NotIncludes>
</And>
</Where>