If you capture a frog, you can kill it humanely by refrigerating it for an hour or two before freezing for an additional 3 hours. A seemingly less humane (but actually quicker) method is placing the frog in very hot, soapy water. 

Alternately, from a University of Florida Wildlife Extension website: "To euthanize a [...Treefrog], hold the frog firmly in your hand and apply a 1 inch bead of benzocaine ointment along the back of the frog. Benzocaine ointment is a topical anesthetic (a numbing agent) used to treat skin pain (e.g., from sunburn) and itching as well as toothaches and sore throats. There are a variety of name brand and generic versions that are available over-the-counter in a tube or spray. If you are able to, using a gloved finger spread the ointment out on the frog's back. Alternatively, you could use a benzocaine spray. Once the ointment or spray is applied, place the frog in a plastic grocery bag or a sealable sandwich bag for 15-20 minutes so that the benzocaine has a chance to render the frog unconscious (be sure to seal the bag or tie it closed). After the bezocaine has anesthetized the frog, place the bag in a freezer overnight to ensure that the frog is dead...".

http://www.wec.ufl.edu/extension/wildlife_info/frogstoads/osteopilus_septentrionalis.php