You can refer to the Syncfusion forum
below for information on how to disable certain options in the dropdown list.
Syncfusion Forum:
https://www.syncfusion.com/forums/163165/is-it-possible-to-disable-the-dropdown-list-in-code?reply=NHPrF1
Also, we already considered the reported
requirement “Support to disable a single items for DropDown components” as
feature at our end and this support will be included in any one of our upcoming
releases. You can track the status of the requested requirement from the below
feedback link.
Feedback link: https://www.syncfusion.com/feedback/23581
To make it count, please cast your vote.
We will prioritize the features based on the demands of each
release. If you have more specifications / suggestions on the
request for the feature, you can add it to the portal as a
comment.