Wrap some twine around them and make a neat finish. Leave plenty of "tail". You can trim it neat later, but you don't want any of the bags to come free because they weren't fastened securely. To make it extra secure, I melt all the ends together by holding them over a candle briefly. DISCLAIMER: fire is dangerous, boys and girls. If you set fire to your house or yourself it is not my fault. When I have done this the bags just sizzle a bit and melt nicely, but if your bags are made of something which bursts into flames or explodes or whatever, don't come crying to me. Be sensible.
It's not too late to join in the Ditch the Disposables challenge. Just pick one disposable product you can do without or replace with a reusable alternative, like pan scrubbers. See the original post for more ideas, or the post about real cloth hankies that started it all, or the one about washable menstrual pads. Or find out what's so bad about disposables anyway? Then vote in the poll in the right-hand sidebar.