It goes without saying; use a Professional Plumber to ensure that you are protected from bad workmanship not only for Material damage, but you are protected from death and serious injury. Below are some images of a T&P (Safety) Valve that was blocked off in 2010 in the KZN Region. The results disastrous to say the least. 

Actual Block off of T&P Valve	Results of the Exploded Cylinder
The affect on the building and Garage door, the brick structure cracked.	The state of the roof, ceiling and structure

The SANS Regulations for Geysers, water pipes and Buildings are there for your Protection. Ensuring compliance and adherence to these standards maintains Quality Plumbing for all. 

The Mythbuster's show us in great detail what actually happens when the safety devices are not utilized on a Hot water cylinder (Geyser). In this snippet you shall see the 200lt Geyser Cylinder turn into a Water Rocket and shoot 152m into the sky - then return to earth. Imagine this shockwave of 100g's going off in your home. One cannot express enough the Importance of COMPLIANCE. Take a look: https://youtu.be/rGWmONHipVo