To stop just some cells being selected use:
cell.userInteractionEnabled = NO;
As well as preventing selection, this also stops tableView:didSelectRowAtIndexPath: being called for the cells that have it set. It will also make voiceover treat it the same as a dimmed button (which may or may not be what you want).
Note that if you have interactive elements in the cell (ie. switches/buttons), you’ll need to use cell.selectionStyle = UITableViewCellSelectionStyleNone; instead and then make sure to ignore taps on the cell in tableView:didSelectRowAtIndexPath:.