Is it possible to compleately and utterly brick a phone with no chance of repair?
Most bricks are "soft" bricks, where the device will not boot but can be fixed by flashing good software.

To fix "hard" bricks requires special tools (like a JTAG setup or USB jig) that are not generally available to end users.  (And yes, it is possible to do this.)

It is also possible to unrecoverably fry the chips on the device, and this could potentially result from kernel development (overclocking and reaching unsafe temperatures, for example).