Realistically, you cannot stop a bad guy from sending deauthentication packets.
Instead, you should focus on ensuring you are resilient to a deauth attack. Make sure your network is configured in a way that the deauth attack doesn't enable an attacker to compromise your network.
To do that, you need to make sure you are using WPA2. If you are using a pre-shared key (a passphrase), make sure the passphrase is very long and strong. If it is not already, change it immediately! If you are not using WPA2, fix that immediately!
The primary reason why bad guys send deauth packets is that this helps them execute a dictionary attack against your passphrase. If a bad guy captures a copy of the initial handshake, they can try out various guesses at your passphrase and test whether they are correct. Sending a deauth packet forces the targeted device to disconnect and reconnect, allowing an eavesdropper to capture a copy of the initial handshake. Therefore, standard practice of many attackers who might try to attack your wireless network is to send deauth packets. If you are seeing many deauth packets, that is a sign that someone may be trying to attack your wireless network and guess your passphrase.
Once the attacker has sent a deauth packet and intercepted the initial handshake, there are tools and online services that automate the task of trying to recover the passphrase, by guessing many possibilities. (See, e.g., CloudCracker for a representative example.)
The defense against this kind of attack is to ensure your passphrase is so long and strong that it cannot possibly be guessed. If it's not already long and strong, you need to change it right away, because someone is probably trying to guess it as we speak.
(The other reason a bad guy might send deauth packets is as an annoyance. However, as most users probably won't even notice, it's not a very effective annoyance.)
To learn more, see these resources: