Can delirium be prevented?

Yes and no. Delirium due to "acute brain failure" as opposed to drug or alcohol withdrawal does have some known and modifiable precipitants. Elderly adults with pre-existing brain atrophy are perhaps the most vulnerable. Several studies have been done in the past 15 years looking at mitigating delirium risk via " delirium prophylaxis". Stanford and perhaps vanderbilt are leaders in this.
Yes. If pt knows the cause of the delirium avoiding it will help. Most common causes of delirium are intoxications or reactions to medications or toxins or to alcohol or drugs.
Risk can be reduced. Delirium has many known risk factors and monitoring for them and intervening early can decrease the risk of delirium. Specifically, avoiding dehydration, constipation, getting enough sleep, assessing and treating infections and other medical conditions promptly are quite important. Older adults benefit from using hearing aids and glasses if needed and being reoriented frequently.