To remove ads on FrostWire Basic, go to:
Main Menu drawer -> Settings and tap on Remove Ads towards the bottom of the screen.
On Remove Ads screen, select one of 3 options and tap on Buy.
Since FrostWire Plus is not available on Google Play there is not a good way for us to introduce an ad removal feature, mainly due to technical limitations.
If you are on Plus, the process is a bit annoying, but easy.
You would have to downgrade to Basic, purchase ad-free subscription following the instructions above, and then install Plus again. The Plus version will read subscription status reported by Google Play and remove the ads automatically when if the subscription is active.