If you mean excess rewards over the $25 cap, they simply aren't paid out and stay in the pool (to be used to pay other post/comment rewards).

If you mean excess rewards in the @sbdpotato fund when no longer needed to support the SBD peg, they will be donated to the Steem DAO aka SPS as stated in the post:

When the @sbdpotato funds are no longer required to support the peg, they will be sent to @steem.dao where the community can then decide what they should be used for.