You can delete a technology that you have created but not researched on the Research tab of the F2 population window. Select the technology as if you were going to research it and then press the Delete button rather than Create. Make sure you are in the correct research field.
If you have already researched it, switch to Completed underneath the technology description on the same tab, which will list all researched projects. Click Remove to set the tech as unresearched and then use Delete to delete it entirely.