I just want to know if the "ATmega8515-8PC microcontroller" of an STK500 can be taken off from the starter kit?

or is it really a default setup that taking it out on the board could cause damage to the starter kit?

 

If it is a 40 pin chip sitting in a socket marked SCKT3100A3 with red surround, on the white area of the board, then I would say yes.

Gently ease a small flat blade screwdriver under each end of the chip but above the socket. Lift a corner at a time evenly and a tiny bit at a time, so that the chip comes up vertically out of the socket.

If you put the screwdriver between socket and board (instead of between socket and chip) then you will likely trash your STK500.

If you fail to lift the chip evenly, then you risk bending the pins. You might be able to straighten slightly bent pins a few times, but not many.

The 40 pin chip in the red socket is just a sample device, not part of the programmer or development board.

Read the manual carefully to see what alternative chips you can fit and the many ways the board can be configured.