That skinny pipe where it is leaking is actually going
into the booster pump. I know it's just semantics, but this way you can understand the direction of the water flow through your system (which is very impressive by the way). I was hoping this "import" connection was a slip fitting attached to a flex line.
It is probably leaking because the PVC is cracked, the thread sealer failed, or just needs to be tightened. But I don't know if you can tighten it because I don't know where the skinny import line is coming from. i.e. is that something that can be rotated or not? Either way, I got the impression you want to just cap it off and do away with it completely. Is that the case or should we keep the option open to reassemble it at some point in the future?
If you want to permanently close it off (and noting is 100% permanent), we make a clean cut on the pipe and glue on a PVC end cap like this:
We would do the same on the outlet side. Although, on the outlet you have a nice fitting which you may be able to get a threaded plug for.
You can also make a clean cut, glue on a cap similar to above but one with a threaded center. Then we thread in a plug into that. (see below)... This is nice because it gives you the option to reinstall a pump in the future with flex pipe and slip fittings.
How comfortable are you with cutting pipe, gluing PVC etc? Want me to get more detailed if this is something you want to tackle?