I am not running ePO 4.5 but 4.0. In 4.0 you can create a query for the subnets not equal to 255.255.255.0. Doesn't look like you can automate in 4.0 since the option is not available to delete subnet. Atleast with the query you can just run the query then select all and choose delete and or whatever option you want. Again this is on 4.0 so check your automation options in 4.5 they could of been updated.
You should take a look and play with automatic responses and you can define what actions to be taken based on creteria created by you.
I am familiar with Automatic responses and uses them to automate many actions, but if you know a specific way to do this with automatic responses please let me know. I can find a way with Automatic Responses to do this