How to ignore columns in parquet/polybase import?
Polybase does not have this ability.  The source file and external table definition must match for it to work.  Simply add or remove additional column(s) in your external table definition.  Then when you are physically creating your table in the database eg via CTAS, add/remove the additional column(s) from your CTAS statement.

Alternately, consider the new COPY INTO statement.  This does have the ability to specify a column list, eg 

COPY INTO test_parquet ( col1, col2, col3, col4 )
FROM 'https://myaccount.blob.core.windows.net/myblobcontainer/folder1/*.parquet'
WITH (
    FILE_FORMAT = myFileFormat
    CREDENTIAL = ( IDENTITY = 'Shared Access Signature', SECRET = '&lt;Your_SAS_Token&gt;' )
)


However please note this feature is i) currently in preview, ii) for copying into physical tables (ie not external tables) and iii) I haven't tested the column list specifically with Parquet.