If an incontinent person with dementia removes or plays with the diaper, you have a few options.
Try using pull-up style incontinence products.These are a bit easier to get used to (because they're styled like regular underwear and come in gender-specific versions. (The person may be removing side-fastening diapers because they're unfamiliar.) Pull-up styles are also harder to take off than side-fastening styles.
Mention this behavior to the person's doctor. It's possible that the person with incontinence is feeling anxious, constrained, or powerless. An antianxiety medication, given for a short period and then gradually tapering off, might relieve these feelings until the person gets used to the undergarments. The medication can be a support through this transition.
Don't give up too quickly. With lots of repetition over time, someone with moderate dementia can still learn to get used to this new reality. At that point he or she may accept the new undergarments and leave them alone.
If the situation doesn't change, you may have to reevaluate the home-care situation and decide whether a more protected living space would be safer. Usually when the person with dementia is removing a diaper continuously, there are other difficult behaviors going on as well.