Unfortunately there is no way to turn off Office Online Server for some web applications in SharePoint Farm, It has to be for entire SharePoint Farm. But you can disable the opening documents in client applications by Default at Site collection level, You can use powershell to do it for all site collection in the Web Applications.
$WebAppsFeatureId = $(Get-SPFeature -limit all | where {$_.displayname -eq "OfficeWebApps"}).Id
$SiteCollection = Get-SPSite -Identity <<SharePoint-Site-URL>>
Disable-SPFeature $webAppsFeatureId -Url $SiteCollection.URL
Thanks & Regards,