The method you want is RemoveUser() which is a tad tricky to use. The method takes 4 parametersRemoveUser (MFAccountAuthorityType, string, MetaFrameAccountType, string)
So Something like
RemoveUser(1,"DomainName",2,"UserName")
Here are some of the enums
# Account Types
$MFAccountTypeUnknown = 0
$MFAccountLocalUser = 1
$MFAccountDomainUser = 2
# Account Authority Type
$MFAccountAuthorityUnknown = 0
$MFAccountAuthorityNTDomain = 1
$MFAccountAuthorityNDS = 2
$MFAccountAuthorityADS = 3
btw... if you are interested I have numerous blog post on managing Citrix with Powershell on my sitehttp://bsonposh.com/archives/tag/citrix