That noise is just part of the beast (it IS the P90s).
I think i got either Pac or MiG to concur that a "noise gate" might actually be a good "solution" to the P90 problem.
The gate kills your signal when it falls below a level that you set.
Given the main problem with the P90 hum noise is that it sounds like bloody hell when you STOP playing, all you really need to do is make sure the gate kicks on when you stop playing, so that the horrible noise is muted.
I'm no guitar tech, but if it isn't currently "shielded", you could shield the cavity of your Strat so that there isn't so much interference.
Your options outside of those two all get in to areas where you are going to end up stripping the P90 sound from your P90s in order to solve the "problem", which leaves you with little whimpy pups in the process.
Try to isolate your guitar from external electrical circuits (i had to learn to stay away from my computer -- think it was the old tube monitor, actually) ... and get a noise gate ... thems would be your two "best" options, along with checking for\installing shielding.
They make something called "hum canceling" P-100s ... but they are known to be somewhat of an oxymoron. (ie. stripping the P90 out of the P90)
- the powers that be left me here to do the thinkin’