Associations are relations between topics. If you don't want an associated topic to appear, you can 'ignore' the topic from your model. Go to the topic of the association that you don't want in the left hand panel and click the X inside a circle which appears when you hover over it. Once you have saved and updated the association will no longer appear in your model.
For example. Say I don't want the association 'monaco':
I scroll to the topic in the list:
If I hover my house inside the blue box, an X appears. Click it. The topic becomes dimmed. After the next update:
The association has disappeared.
Note that it is also possible to change topic titles to something more informative, or to merge them with each other to create larger subsuming topics.