First, dropdownlists have a remove option. Something like
ddl.Items.Remove(ddl.Items.FindByValue("value"));
Another way would be to just not select those items from the list. This refers to you question to "hide" database values after retrieving. I take you use DropDownLists something like below:
DropDownList1.DataSource =ds.Tables[0]; // you list of items retrieved from DB
If you only like to display a subset of this table just retrieve only those records you need from your database using whatever DB connection options you have. For instance after the user selected OK use something like pseudo code below:
// get full list
allItems = DB.getAllItems();
selectedItems = allItems.where(p => p.itemID != itemID1 || itemID2);
DropDownList1.DataSource = selectedItems;
And for the rest specify the rest of the required values to init the dropdown.
Or in other example code, something like:
SqlCommand ddSqlCommand = new SqlCommand("SELECT * FROM TableName
WHERE ID <> ID1 AND ID <> ID2", ddSqlConnection);
ddSqlConnection.Open();
ddDR = ddSqlCommand.ExecuteReader(CommandBehavior.CloseConnection);
DropDownList1.DataSource = ddDR;