What version of the product do you use? This sounds vaguely familiar. Please clarify your question and describe in detail what you are doing -- it sounds like you are just referring to something which occurs automatically in the worksheet.
In terms of SQL Developer features that may need to deal with schema names (like Tools > Database Copy/Diff/Export), we offer options or preferences to include / exclude the schema name:
1. Tools > Database Copy... > Copy Options > Objects Copy | Schemas Copy (Maintain | Consolidate)
2. Tools > Preferences > Database > Utilities > Difference > Diff DDL Generation Options > Schema checkbox
3. Tools > Preferences > Database > Utilities > Export > Export/View DDL Options > Show Schema checkbox
sounds like you're using the code insight to get a table list for your current schema, and the auto-complete nukes the schema. portion of your query?
that's an 'automatic' feature, i don't think there's a way to disable that, of course you dont' have to use the completion insight feature, if you type it out, nothing will get removed
thanks Gary Jeff nailed it below.