1. Go into configs, and find the config file that is for mmmPowersuits. Open it for text editing, and under the properties section find "D:"Plasma Cannon.Plasma Explosiveness.Voltage.multiplier"" and set it equal to 0.0, that will nullify it's explosive effect on EVERYTHING.
2. To disable rejuvenation explosives, we do the same thing. Open the config folder, then open the Universal Electricity folder. Find the config file called ICBM and open it for text editing, the second section (the one under crafting) should contain "B:"Disable rejuvenation"" and set it equal to true.
Since you disabled the rejuvenation explosives, it probably wouldn't be nice to allow people to craft it since it would be a waste of resources, so under the crafting section find the values "B:"Allow rejuvenation Crafting"" , "B:"Allow rejuvenation Minecart Crafting"", and "B:"Allow rejuvenation Missile Crafting"". set them all to false.
Follow those steps, and the plasma cannon will not do harm to any blocks, and disabling certain explosives (ie. RedMatter, Ultrasonic, etc.) is a breeze.