If you are referring to the long metal tube...yes you can cap that off. But be aware that you will run into over heating issues because that pipe carries hot water from the cylinder head back to the thermostat which allows the thermostat to open when you first crank your car. Otherwise, your coolant will not circulate and the engine over heats.
If you delete that tube, you will either have to drill holes through the tstat or just delete the tstat all together...using a "low temp" t stat will not fix the problem because the t stat will not see hot enough temps to open without that flow circulating back from the head.