Unless the item is no longer sold to any customer so that you can deactivate the item, it can not be hidden from view in Demand Planning.
One helpful way to hide these non-forecasted items is to use the Forecast Type value that gets added to demand plan line when the item demand plan is edited or turned off. When the customer/item demand plan is edited it is tagged with a UDF (user defined forecast) forecast type. When the forecast is disabled, the customer/item demand plan is tagged with a DNF (do not forecast) forecast type. The forecast type can then be used to filter the line from view in reports.
The forecast type can also be added as a field when preparing a custom report.