The BBEG speech is a staple of the genre, albeit unfortunately one that doesn't stand up to too much IG logic--after all, once the PCs have come upon the BBEG, their logical next step would be to attack immediately before he has time to summon guards, activate traps or otherwise decrease their chances of success. After all, they can (theoretically) always question him later. If a BBEG has the actual power to force all of the PCs to stand there passively while he gives a speech, he's probably too tough to fight.
But, it's a staple of the genre, so let's try to make it work.
You need to look at how you are incentivizing the players. Your current system gives an incentive for a player to interrupt the BBEG speech, and a pretty large one at that. If your desire is to actually include BBEG speeches in your game, this is a mistake. (Beyond that, it's an incentive found nowhere in the rules, and I'm a little perplexed as to why you would make up a house rule to encourage behavior you don't want to happen.)
You next need to look at whether or not you have been giving other incentives to interrupt. For example, in the past, have players been punished for sitting there patiently listening to the speech by having the BBEG summon guards, activate traps or set up some magic spell/device at the end of his speech? If so, then you're training your players in exactly the opposite direction of what you claim you want.
Here's the easy solution: Interrupting the BBEG causes combat to start, with normal initiative being rolled. The player who chooses to interrupt gets no special benefit (the IG explanation for why this is the case is that, like a gunslinger in a shoot-out, whoever rolled higher than the person trying to interrupt saw the putative interrupter start to move and beat him to the draw).
You then explain to the players: "Okay, guys. If you don't let the BBEG talk, we all roll initiative and fight. If you do let him finish talking, we all roll initiative and fight. There is no benefit to not listening to the boxed text, and I will not punish your characters for doing so."
(As a final issue, you should probably examine your BBEG speeches. Sometimes, the reason why players interrupt is that they're tired of sitting around listening to the GM talk to himself. As the players are indicating by their interruption that they think that sitting around listening to your speech is a waste of time, you should probably examine the speeches to see if they're right.)
At that point, if your players are still interrupting your BBEG speeches, then either you need new players or you need to join an amateur theater troupe to get the urge to soliloquize out of your system.