Well, as a quick fix you can either:
Add this to the control on the report, under it's 'Control Source'
- =Trim(Left([YourFieldName],InStr([YourFieldName],"-")-1))
or
Go to the report's recordsource, and in design view add in a new field expression, like:
- NewFieldforReport: Trim(Left([YourTableFieldName],InStr([YourTableFieldName],"-")-1))
Just replace the data in the brackets with your actual names.