public class RoomWarningSwallower : IFailuresPreprocessor
{
    public FailureProcessingResult PreprocessFailures(FailuresAccessor failuresAccessor)
    {
        IList<FailureMessageAccessor> failList = new List<FailureMessageAccessor>();
        // Inside event handler, get all warnings
        failList = failuresAccessor.GetFailureMessages(); 
        foreach (FailureMessageAccessor failure in failList)
        { 
            // check FailureDefinitionIds against ones that you want to dismiss, 
            FailureDefinitionId failID = failure.GetFailureDefinitionId();
            // prevent Revit from showing Unenclosed room warnings
            if (failID == BuiltInFailures.RoomFailures.RoomNotEnclosed)
            {
                failuresAccessor.DeleteWarning(failure);
            }
        }

        return FailureProcessingResult.Continue;
    }
}

Public Class RoomWarningSwallower
    Implements IFailuresPreprocessor
    Public Function PreprocessFailures(failuresAccessor As FailuresAccessor) As FailureProcessingResult Implements IFailuresPreprocessor.PreprocessFailures
        Dim failList As IList(Of FailureMessageAccessor) = New List(Of FailureMessageAccessor)()
        ' Inside event handler, get all warnings
        failList = failuresAccessor.GetFailureMessages()
        For Each failure As FailureMessageAccessor In failList
            ' check FailureDefinitionIds against ones that you want to dismiss, 
            Dim failID As FailureDefinitionId = failure.GetFailureDefinitionId()
            ' prevent Revit from showing Unenclosed room warnings
            If failID = BuiltInFailures.RoomFailures.RoomNotEnclosed Then
                failuresAccessor.DeleteWarning(failure)
            End If
        Next

        Return FailureProcessingResult.[Continue]
    End Function
End Class