Came across an issue removing some really old citrix clients (version 7.x) in a locked-down environment that was running (ahem) windows vista machines. Had issues upgrading the receivers on the machines.
As these machines had ICA clients using a version that was packaged using installshield they used an EXE file. The method to remove these files was somewhat different than the later MSI method. To remove successfully:
isuninst –f C:\PROGRA~1\Citrix\ICACLI~1\Uninst.isu –c C:\PROGRA~1\Citrix\ICACLI~1\uninstpn.dll –a.