Import-Module 'C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.dll'
$siteUrl="Your site collection URL"
$webName="Your web name(sub site name)"
$groupName="Your group name "
$spSite=New-Object Microsoft.SharePoint.SPSite($siteUrl)
$spWeb= $spSite.OpenWeb($webName)
$groupCollection = $spWeb.Groups
$groupCollection.Remove($groupName)
$spWeb.Dispose()
Take http://www.contoso.com:8899/sites/shelley as an example, its subsite is IT, HR, Accounting which have a common group: Administration along with full control permission.
Make sure they have stopped the inheritance from the parent, execute the following script:
Compared with http://www.contoso.com:8899/sites/shelley/IT, HR site has not Administrator Group Permission, but IT Site still has Administrator Group Permission.
HR Site IT Site
Hopefully, it can help you resolve the related issue.