Solution:
Add implicit style to the page where the ComboBox is defined:
<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:telerik="http://schemas.telerik.com/2022/xaml/maui"
x:Class="MauiApp5.MainPage">
<ContentPage.Resources>
<ResourceDictionary>
<Style TargetType="telerik:RadEntry">
<Setter Property="IsSpellCheckEnabled" Value="False"/>
</Style>
</ResourceDictionary>
</ContentPage.Resources>
<VerticalStackLayout>
<telerik:RadComboBox ItemsSource="{Binding Items}"
DisplayMemberPath="Name" x:Name="combo"/>
</VerticalStackLayout>
</ContentPage>
Regards,
Didi
Progress Telerik