Can we tell dbcc to ignore subnormal real values?
The whole purpose of DBCC CHECKDB WITH DATA_PURITY is to check the database for column values that are not valid or are out-of-range.

In sql server 2000, it was possible to store out-of-range values in columns of certain datatypes like Float,real, datetime, etc. But moving from SQL 2005 and up, SQL Server will flag out those out-of-range values as it follows Single-precision floating-point format. Check @PaulWhite's answer here.

From BOL :


  DBCC CHECKDB detects columns with date and time values that are larger than or less than the acceptable range for the datetime data type; or decimal or approximate-numeric data type columns with scale or precision values that are not valid.


You must fix those errors reported as described in KB923247.