I've only just noticed this and was shocked to find that things have got so bad before you sought help.
Don't worry Kate, the situation is serious but with your help, Leon can be brought back from the brink.
The main problem is that, once the trek is in him, it will never completely removed - as Gilly said, "He is not the person you once knew".
With this in mind, the only way forward is for Parameters to be set regarding Trekkie activities in order to minimise the impact on both your lives.
Watching of Star Trek must be limited to episodes featuring the following women:
The model making must cease immediately before he buys a comfy cardigan and zip up slippers.
Action figures of the above women is permissable but they must not be used to re-enact favourite episodes.
Similarly, magazines and books must be limited to a few select copies containing the same female characters.
The acting out of Trek based fantasies is acceptable as long as they involve norks and the exchange of bodily fluids.
Throughout the above conditioning, vigilance is essential on your part.
If you hear him utter the word "convention" or if he starts speaking in a guttural language or holds up a hand with the figers separated between the middle and ring fingers saying the phrase "Live long and prosper", a swift kick to the nads will teach the unfortunate man that such behaviour is unacceptable.
I wish you good luck in your attempts.
Stan
During research for the above, I had to search google images using the word trek. Some of the images I had to view have traumatised me to the extent where I will now need to perform similar searches using words like norks for at least the next half hour.