it flashes on then immediately dies as soon as I plug it in to any power source
Although you are probably out of luck since it is intended to protect against over current, not over voltage (over current would occur, e.g., if you tried to draw too much via the breakout pins), there is always the chance that the main polyfuse triggered, since the problem with excess voltage is that it produces too much current.
So, you could set that Pi aside and see if it recovers (from other reports we've had here, a few days should suffice) but again, probably not.
Could voltages over 5v damage a pi?
The recommended maximum is 5.25V, and yes anything in excess of could be catastrophic for the device (although as per joan's comment below, you may actually get away with somewhat more). The problem is that current is a product of voltage and resistance (Ohm's law). The resistance is engineered in the device's design to deal with a specific, narrow range of voltage (in the Pi's case, 4.75 - 5.25V).
If you supply less than that, there is too little current and the device will not work.
If you supply more than that, the amount of current will increase correspondingly. Excess current will cause components to heat up, and at a certain point, that heat will physically damage them. Polyfuses excepted, such damage cannot be undone.
I believe the polyfuse is less likely to protect against overvoltage because more delicate components in line after the fuse will heat up to a critical point first. However, the big picture is not necessarily that simple (see Ghanima and Milliway's comments below regarding the presence of a TVS diode in the 5V power regulation circuit; above a certain threshold, this will mean voltage will flow to ground with little resistance, causing a surge in current which hopefully trips the fuse), and it does not hurt to cross your fingers.
I measured one such usb adapter at 7.5v
That is probably defective, since almost anything plugged into a USB port is going to be expecting 5V. Considering they are often used to recharge batteries which can explode, it is a potentially dangerous defect, so you might want to give some thought to where they came from.