READ THE DATASHEET.
The Max input voltage for an LM7805 is 18V, not 24V so the heatsink isn't going to do squat.
You could use a power dropping resistor to dissipate 24-7.5=16.5 V at 1.5A = 25W. (24.75)
Rdropping = (Vin - VLoad)/ ILoad = 11 ohms
PRdropping = IRdropping x VRdropping = 24.75 W
Would it be better to use a switching dc-dc converter (like the LM2596) from ebay ? (probably)
Do you have time to wait for that to arrive ? (I don't know)
Can you use the resistor until it arrives ? (yes)