You can not disable regeneration in assembly completely. Even if you never push Regenerate button various commands like Edit Definition, Edit References, Suppress / Resume will trigger some kind of assembly regeneration. This is done in order to keep dependent components (child components) up to date with parents changes.
"High level" regeneration (usually of components and assembly features) happens upon assembly retrieval as well - if some changes are detected. Opening simplified representation should minimize this type to almost zero.
If you want to avoid certain components / features from regeneration you have 2 ways :
1. Set respective assembly / part features (all features) to Read Only status. You will also be unable to edit them
2. If you only want to eliminate regeneration triggered by parent changes you can use so called "update control" techniques - create geometry backups for respective components or features and set them to "Manual Update" status. This can be done to entire assembly as well.