I posted awhile back about how Chloe basically adopted Theo and Delilah and let them "nurse" even though she's never had a litter of her own and obviously wasn't producing milk. The kittens are about 16 weeks old now and they're still doing it. Is there any way I can make her make them stop? She keeps encouraging them to do it.

The only thing I can think of is try to feed them more kitten food...maybe if they are too full from that they won't bother her as much.
Here are a few sites I looked up for you. :)
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Got this from another site: I hope all this helps :)

Many times, weaning is the mother cat's responsibility. When the kittens are 4-5 weeks of age, mom will begin to refuse nursing. This encourages the kittens to eat solid foods. As part of the process, mom will resume nursing them periodically. In a few weeks, the kittens should be completely weaned.

It is understandable that your kittens want to resist weaning. Try placing warm kitten formula alone, in a dish for them. Slowly place foods that add texture, progressing to tuna. Kittens and cats all love tuna and I have found this to be one of the more effective foods in weaning. As a final step you may want to physically separate the kittens from mom cat. This will not only give mom a break but will also help break the kittens' dependence. Keep an eye on mom and take her to the vet if become concerned for her health. Let me know how things go.

The same thing happened with Keiko and Pixelle and they stopped by themselves... At this point they don't do it because they're hungry but for comfort... I wouldn't worry about it... Chloe will know when it's time to stop...