Any fast way to clear a table?

   Unfortunately, no.  Using DevForce, you need to first load all those entities into cache, and then either Delete() them or ensure that a cascaded delete affects them.  

If you don't need to treat the data as entities and only need a means of performing bulk operations on the database, you can use the AdoHelper in IdeaBlade.Rdb to drop into ADO.NET.  It's a bit of a convoluted path to get there, but it's something like this:

      IDataSourceKey key = myEntityManager.DataSourceResolver.GetDataSourceKey("Default");

      // cast or use appropriate EdmKey constructor
      IdeaBlade.EntityModel.Edm.v4.EdmKey edmKey = dataSourceKey as IdeaBlade.EntityModel.Edm.v4.EdmKey;

      RdbKey rdbKey = edmKey.RdbKey as RdbKey;
      AdoHelper helper = rdbKey.AdoHelper;

      // Can now create DB connection and commands directly.
      var command = helper.CreateDbCommand(..);