Officially, interrupting meditation or study results in no spells prepared...

Each of the classes you mention has no provision for partial preparation of spells.

• "Each cleric must choose a time when she must spend 1 hour each day in quiet contemplation or supplication to regain her daily allotment of spells."
• "A druid must spend 1 hour each day in a trance-like meditation on the mysteries of nature to regain her daily allotment of spells."
• "[A wizard] must choose and prepare his spells ahead of time by getting 8 hours of sleep and spending 1 hour studying his spellbook."

Using a strict reading of the rules as they are written, that's exactly how long spell preparation takes, and stopping early means no prepared spells. Yes, the day after the massive battle that exhausted all his spells, even Count Slimeheart, Archmage of the Drooling Tower, if interrupted 1 min. before finishing his hour of study, will find himself with no spells prepared.

...And sorcerers neither meditate nor study...

Sorcerers lack the problem their more devout and studious compatriots possess:

Unlike a wizard or a cleric, a sorcerer need not prepare her spells in advance. She can cast any spell she knows at any time, assuming she has not yet used up her spells per day for that spell level.

A sorcerer wakes up in the morning and is ready to go. He's unstoppable! (Of course, there's also the 50/50 chance he's 1 entire spell level below other full casters, but that's beside the point.)

...So the only recourse to prevent a sorcerer from being ready is making a house rule

If made at the campaign's beginning, a house rule mandating that the sorcerer (and similar casters like the bard) must spend an hour upon awakening in meditation or whatever is reasonable. Although such characters often have hard enough lives, such characters are also usually full casters, so it's not as damaging a house rule as, for example, a house rule that somehow nerfs melee attacks. It's a burden, certainly, but it's not an excessive burden so far as house rules go.

However, a GM who makes such a house rule mid-campaign then attacks the party of bards and sorcerers while they're meditating is really just asking his players to leave.

An Aside: Please don't attack PCs while they're preparing spells. If you do, they'll try to do the same to the bad guys. You don't want to spend a whole afternoon making a wizard baddie only to have the baddie dead before his turn because the PCs interrupted him while he was studying, so don't do that to the PCs your players have been tinkering with for months either.

If you're afraid of the PCs downing the big bad evil guy early, don't use him! Send instead paid assassins, bound or allied creatures, created minions, simulacra, projected images, or fanatics. Have the baddie mail threatening letters containing a magic mouth or explosive runes. While the PCs aren't around, have the villain explode their home towns, frame them for crimes, damage their reputations, hurt their moms, ruin their NPC friends, and eat their pets. Sending the BBEG into harm's way means the PCs should have a chance to end him. (And any method the GM uses to save the BBEG should eventually be accessible to the PCs!) If the only way to further the plot is a house rule that mandates sorcerers meditate, it might be time to consider—sadly—a different plot.