Disable autostart for a service without uninstalling?

This should do the trick:

Open terminal (Ctrl + Alt + T) and enter the following command:

sudo update-rc.d tomcat disable

Basically update-rc.d will modify existing runlevel links for the script /etc/init.d/tomcat by renaming start links to stop links.

More generic and more visual, with a nice UI: sysv-rc-conf

Uncheck the boxes for tomcat7 (runlevels 2 to 5), quit and that's it.

The disable|enable API is not stable and might change in the future. I suggest you use the following command to remove all the symlinks in /etc/rc?.d/:

update-rc.d -f tomcat remove