If you don't see that the feature would fit your vision of the codebase then just tell them that.
If you think it is a useful feature tell him that you currently don't have time to implement it yourself but a PR is welcome.
Often people only want that there issue is tracked, so to avoid conflict and just do nothing just tell them that it goes into the backlog of future features to be considered ;)
Just another point to consider: sometimes reviewing a PR and getting it up to your standards is significantly more work than doing it yourself.
If you don't have time to implement it, there is a chance you don't have time to review it.
If it is like that, you can simply say you don't have the bandwidth to review, and/or you don't want the feature in your repo. You can emphasize that they can always simply fork the project and do whatever they want with it.
If you think it is a useful feature tell him that you currently don't have time to implement it yourself but a PR is welcome.
Often people only want that there issue is tracked, so to avoid conflict and just do nothing just tell them that it goes into the backlog of future features to be considered ;)