You could override the RecognizesAccessKey property of the ContentPresenter that is in the default template for the label. For example:
<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Grid>
<Grid.Resources>
<Style x:Key="{x:Type Label}" TargetType="Label">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Label">
<Border>
<ContentPresenter
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
RecognizesAccessKey="False" />
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Grid.Resources>
<Label>_This is a test</Label>
</Grid>
</Page>