If you have ArcGIS 10 and an ArcInfo license, you can try the Eliminate Polygon Part tool (Data Management toolbox - Features toolset), which allows you to remove contained parts/holes.
Also, Feature To Polygon tool (Data Management toolbox - Features toolset) produces polygons from lines and/or polygons. Since your final polygons represent some classifications, you can try the following:
1. Select the lines or buffer polygons of the same class;
2. Run Feature To Line to get polygons (spaces surrounded by lines or polygons will become polygons);
3. Run Dissolve tool to remove interior boundaries.
4. Repeat 1 - 3 for each class. Then Merge or Append them together.
It might be necessary to write a script or build a model to do this in a loop/iteration. You may still end up having holes/spaced between classes. I am not sure which class those spaces should belong to. You will need sort that out.
Hope the above helps to some degree.
Regards,