short anwer is you cant. if any of those strings and maps inside the class arent connected to any other part of the app, eventually they will be cleaned by the garbage collector. and by eventually i mean, it could simply not happen if the garbage collector doesnt want to, may be there is too much free ram.
if you are running into issues of memory, then it means that all those 1000 classes have connections that are preventing from being cleaned. that connection is called memory leak.