Per Maryland Form 502 Instructions, page 2:

Baltimore City Residents:

Leave the Maryland County box blank. Write "Baltimore City" in the CITY, TOWN or TAXING AREA box.

In order to correctly calculate the Baltimore City Tax Rate, the County field in the address section must be left blank. In order to do this, please select None Selected in the County field. This will calculate a blank entry in the Maryland County box on Form 502. If None is entered in the Maryland County field this will calculate a tax rate of zero for Baltimore City.

To enter the information in TaxACT:

1. From within your TaxACT return (Online or Desktop), click on the State Q&A tab
2. Click Maryland directly below the blue tabs
3. Click Basic Information to expand the category, and then click County and City
4. Select None Selected from the drop down list and click Continue