How to hard reset of a Samsung Galaxy GT-I9000 which I cannot put in recovery mode?
Did you try this:


Turn the power off. If your Samsung Galaxy S is frozen, pull the battery out and reinsert it
Hold the Volume Down button
Press and release the Power button
You are now presented with a menu that allows for Fastbook, Recovery, Clear Storage, and Simlock
Select Clear Storage by pressing the Volume Down button
Press and release the Power button
Now simply confirm your decision: Volume Up for YES and Volume Down for NO


(Source: KnowYourCell.Com -- the first hit on a little Google Search)



UPDATE: Alternative approaches are listed at Hard-Reset.COM, e.g.:


charge your battery fully before doing a hard reset
PRESS AND HOLD VOLUME UP + Home Button (or VOLUME UP + DOWN)
Keep pressing these 2 buttons + press Power button for 2-3 Seconds
Release only power button, but KEEP pressing Volume up + Home Buttons (or VOL UP + DOWN)




You also might check with the following question here: SGS doesn't go to download, recovery mode nor does it connect via USB after installing a battery MOD