Hi Eva,
If you want people to be able to watch the video for reference but NOT to use the video in a performance context, then I think you have a couple of choices:
1. You can put it on Vimeo, make the link password-protected, and in the settings you can stop people from downloading it or embedding it in other websites.
However, they could still stream the video live.
2. Watermark the video. This is, I think, the only real way you can stop the video being used. Make the watermark big enough and central enough, or perhaps make it repetitive across the whole video, so that if it is seen it is very clear. In the past I have used a phrase like "not for public use" and put it big and bright across the centre of the video.
3. Give them a low resolution version of the video that would look awful if shown full size.
4. A combination of 3 and 4.
Something which I've gained from experience: at the beginning of the project, even before the contract is drawn up, establish who owns the intellectual property (the IP). I try and fight for my ownership of the IP, as artist, as much as I can but it's not always possible, so sometimes I have to contractually share my Isadora patches and all content that I have created. Understanding who owns the IP is crucial in situations like yours, Eva. If you clearly own the IP including the videos that you created, then you have legal rights if 'they' try and use the video without your permission.
HTH
Mark (not Mark)