Set "never combine" in Windows 7 using the registry?

You're looking for the "never group taskbar buttons" feature.

HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced\TaskbarGlomLevel = 00000002

See KnifeySpoony's comment on an article about Windows 7 taskbar buttons at HowToGeek for more information.

TaskbarGlomLevel – Changes the grouping so that the windows do not turn into squares and overlap each other. They will still group if you have too many windows open. If you want to never group windows, change this value to 00000002.

Standing on the shoulders of Rob:

Via powershell (src):

#http://superuser.com/questions/135015
$taskbarButtonsRegKey = 'HKCU:\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced'
if (((Get-ItemProperty -path $taskbarButtonsRegKey ).TaskbarGlomLevel ) -Ne 2)
{
    Set-ItemProperty -Path $taskbarButtonsRegKey -Name "TaskbarGlomLevel" -Value 00000002
}

Via chocolatey (package):

choco install taskbar-never-combine