It is currently not possible to disable Dynamic Synonym Suggestions for an index. If you do not want to use them for that index, you can simply ignore them.
If you do not interact with the Dynamic Synonyms feature for 90 days (ie: accept/reject a suggested synonym, update your configuration, etc), then the engine will automatically stop suggesting synonyms
For a full list of common synonym issues, troubleshooting topics and best practices, see our summary article here.