Sub RemoveCustomFunction( _
   ByVal name As String _
) 

Dim instance As ICustomFunctionSupport
Dim name As String
 
instance.RemoveCustomFunction(name)

void RemoveCustomFunction( 
   string name
)

public class CubeFunctionInfo : FarPoint.CalcEngine.FunctionInfo
    {
      public override string Name { get { return "CUBE"; } }
      public override int MinArgs { get { return 1; } }
      public override int MaxArgs { get { return 1; } }
      public override object Evaluate (object[] args)
      {
        double num = FarPoint.CalcEngine.CalcConvert.ToDouble(args[0]);
        return num * num * num;
      }
    }

DialogResult dlg;
FarPoint.Win.Spread.Model.ICustomFunctionSupport cfs;
cfs = (FarPoint.Win.Spread.Model.ICustomFunctionSupport)fpSpread1.ActiveSheet.Models.Data;
cfs.AddCustomFunction(new CubeFunctionInfo());
fpSpread1.ActiveSheet.SetFormula(1, 1, "CUBE(4)");
dlg = MessageBox.Show("Do you want to remove the custom function?", "RemoveCustomFunction", MessageBoxButtons.YesNo);
if (dlg == DialogResult.Yes)
{
     cfs = (FarPoint.Win.Spread.Model.ICustomFunctionSupport)fpSpread1.ActiveSheet.Models.Data;
     cfs.RemoveCustomFunction("CUBE");
     fpSpread1.ActiveSheet.RecalculateAll();
}

Public Class CubeFunctionInfo
   Inherits FarPoint.CalcEngine.FunctionInfo

   Public Overrides ReadOnly Property Name() As String
        Get
             Return "CUBE"
        End Get
   End Property

   Public Overrides ReadOnly Property MinArgs() As Integer
        Get
             Return 1
        End Get
   End Property

   Public Overrides ReadOnly Property MaxArgs() As Integer
        Get
             Return 1
        End Get
   End Property

   Public Overrides Function Evaluate(ByVal args() As Object) As Object
        Dim num As Double = FarPoint.CalcEngine.CalcConvert.ToDouble(args(0))
        Return num * num * num
   End Function
End Class

Dim dlg As DialogResult
Dim cfs As FarPoint.Win.Spread.Model.ICustomFunctionSupport
cfs = FpSpread1.ActiveSheet.Models.Data
cfs.AddCustomFunction(New CubeFunctionInfo())
FpSpread1.ActiveSheet.SetFormula(1, 1, "CUBE(4)")
dlg = MessageBox.Show("Do you want to remove the custom function?", "RemoveCustomFunction", MessageBoxButtons.YesNo)
If dlg = DialogResult.Yes Then
     cfs = FpSpread1.ActiveSheet.Models.Data
     cfs.RemoveCustomFunction("CUBE")
     FpSpread1.ActiveSheet.RecalculateAll()
End If