Civilization II Design 6. Thou shalt prepare for future wonders well in advance.
Some wonders are just too good to allow anyone else to enjoy.
You can't prevent some poaching, but you can make it harder for them. When on the verge of an advance that will let you build a particular wonder, start stockpiling those caravans and freight units. As soon as you can, rush those camels to the build site and raise the rafters pronto. If your foes beat you to the punch, and you've got other wonders available, switch over and build those rather than take the 50it on switching production. It is often better to keep a city building a wonder that is already built rather than trashing the shields, as long as you can roll the production over into another wonder fairly soon.