I think you need this:
SET @SQLString = N'spiTriggerControl';
to be this:
SET @SQLString = N'Exec spiTriggerControl';
I'm not sure why you are using sp_executesql based on the information you have provided. You could just do:
DECLARE @Trig_ID_Out VARCHAR(10);
Exec spiTriggerControl @Trig_ID_Out OUTPUT;
SELECT @Trig_ID_Out