[Sitraahra]

I once had a character who was a fallen angel..and was addicted to drugs and sex as a means of helping him forget heaven.

but anyway, if you really think you can't balance out the character, or don't want to deal with it, and are really afraid it'll be too powerful, just tell your player that you're not comfortable with that. Gm's can be human too

however, I find it mroe interesting to let them play it out with negatives. You want to be an archangel? well you're part of a divine order you got to obey, and remain faithful to all its laws. It doesn't have to actually hurt the player to be balanced, just restricting. Or an ECL...say, +10? If you choose to go this route, always go with the more restricting choice [if they palyer doesn't like it, he doesn't ahve to play the modified race], and if they add something new to the mix, consider it carefully on its own

though, the most annoying powergamer I know seems to have adjusted to this [ and not just with me]: he asks for something simple, like changing the way vampires work, and then adds a normal spell or magical item to make it overpowered

[super sorcerer]

In our group it is well known that you can't have yoour own self invented items unless you have the required item creation feat and you know spells that make similar enough effects. If someone is willing to spend a huge amount of XP and make a "blinding greatsword +3" (which is a greatsword that cast power word blind on every hit) than he is invited (the market price become 3²*2000(+3 enchantment)+13*7*2000(spell level7 caster level13 use activated)=18,000+182,000=200,000(exactly enough so it won't be epic) for a total XP cost of 200,000/25=8000XP). After all he must take the craft arms and armor feat, learn powerword blind and spend 100,000 gp 8000 XP and 200 days for that luxury greatsword.
About races nobody is willing to take level adjustment in our group becouse it take too much. Once someone wanted to play a planetar angel on m campain (when I was the DM) , I gave it a level adjustment of +3 (ECL 17 for the CR 17 I would give to an elite planetar) and it realy wasn't too powerfull.
I never met a player who just wanted things for no good reason. For starting equipment we usualy use only things from the DMG although anyone with item creation feats may spend as much XP and gp as he want to make his own items before the game start.

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though, the most annoying powergamer I know seems to have adjusted to this [ and not just with me]: he asks for something simple, like changing the way vampires work, and then adds a normal spell or magical item to make it overpowered

Never make rule changes unless either you checked every possible option to abuse it or it is the 3E rules and you are using 3.5E rules (unless it is something horribly unbalancing on 3E). For example I use the weapon system of 3E becouse it make much more sense to me (how a small lgreatsword could be any different from a longsword?) and anoter DM used the 3E rules fo spell focus in the forgoten realms campain. Make shure it is unabusable before you make any change (espeacialy when a player suggest these changes).

We never have to say no becouse we never ask for a keen vorpal dancing greatsword +20 on level 1. If someone want to get such thinhs then saying no is the only reasonable answer. I don't understand why do you try to find other solutions to problems that usualy never rise with reasonable players. Anyway we allow the PCs to do anything the NPC do and vise versa so if we allow any munchkinism to the PCs the NPCs are also allowed and it just ballance it.