If buying an animal be very careful when considering your options. Don't buy a pet that is likely to outgrow the space you have available or outlive the likely interest of its carer.

Don't release unwanted pets into the wild. Such action is illegal, can threaten native wildlife and is often not the best option for the pet involved.

National and international strategies dealing with non-native species rely on, amongst other options, detection and surveillance. So the best thing to do is to report your sighting. 

Control of non-native species should be undertaken only after consideration of the risks posed by the species in question and balancing these against the cost effectiveness of any proposed action. In the case of the North American bullfrog, the risk is judged to be high.  Immediate action should be taken to confirm the species identification and remove any such animals from the wild. In the case of most other non-native amphibians and reptiles, the risks are not fully understood - but learning about where these animals occur will help with ongoing assessment. So, in all cases, please report your alien sightings.