Are we talking about an aftermarket alarm or a factory keyless entry? I do not know of any factory GM keyless entry units that honk the horn so I'll assume that it is aftermarket.
The more modern alarm systems recognize the transmitters by programming the "brain" for each transmitter code rather than programming the transmitter to operate the brain. This is to say that the transmitters have a code associated to them and the brain learns this code. Most systems will have 4 or so programming "slots" to use multiple transmitters. If you in fact to have an aftermarket system then I would reprogram the transmitter codes again, filling up all of the slots. This way, any previous stored codes will be deleted.
For instance, if your system has the capibility of learning 4 transmitters and you only have two, program them both twice.
More advanced theives have scanner type devices that are able to read your transmitter codes when you use them. Later, this code can be reproduced to disarm/unlock your vehicle. This is the reason for the newer code-hopping technology that makes the scanners useless because the transmitters send out a different code each time.