I mean that they have no research points left, no research points possible, and the so-called "useless" researchs may be stupid to research, yet some people research them anyway. So, the only absolutely not option limiting idea would be to add the sent over researches free of research point cost, and that has obvious abuse.
I think there are problems associated with this. However one way of doing it is as follows:
* When sending a Colony Ship a button is available "Sent Techs" which allows up to (eg) 20 points of technology to be sent, with a maximum of 5 techs being sent.
* If the CS arrives successfully and conquor concludes successfully then the techs at the destination learn the new ability and the techs from the source are successfully sent home.
* If the CS is destroyed then the Techs are lost, meaning that that technology is lost from the source city (ie: only those techs know the technology). In academy there is then an option under reset "Lost Techs: <number> <Reset button>". Pressing reset button gives a 30 second cool off (to avoid multiple clicks by accident). Player would then have to research the lost technologies in source city again.
* During the travel & occupation time the technology should not be available to source and should be red in the source (& can not be reset). Travel & Occupation is the time from CS being sent until the conquor concludes or the CS arrives home (if failed but not destroyed).
The thing is that the source player wont know when sending what the destination city has, so it would be a guess and a risk. Would you risk important technology from your source?
I also believe that when a city is conquored that the academy should have no technology researched (excluding any done using this). My reason is because any techs in the academy would be killed in the battle. So you it then makes sense that you must take over your own techs.