IsLinkable is set by the author of the DataSource code, to indicate whether a data source type as a whole can be linked.
You're looking for the DataTableSaveSettings class. Somethign like the following code snippet should suffice:
DataManager manager = table.Context.GetAncestor<DataManager>();
DataTableSaveSettings settings = null;
if (manager.SaveSettings.TryGetDataTableSettings(table.Id, out settings))
{
settings.UseLinkedData = false;
}
else
{
settings = new DataTableSaveSettings(table, false, true);
manager.SaveSettings.DataTableSettings.Add(settings);
}