The horn will not go off after jumping the battery

We I just bought this car and it had to be jumped last night. When I went to go jump it the alarm went off. I tried closing and locking the doors but it did not work. Then I tried pulling the fuse to the horn, but it did not work either. I would have just let it charge but it was already 8:00 pm and the neighborhood is quiet so I did not want to disturb. What can I do to prevent the horn to go off?