Thank you all for the responses. I think there are some good suggestions on solving this maybe in future releases. The idea, that a CC in nomad mode is built instantly without the ability to be blocked by an enemy unit would definitely solve the problem. Making it invisible is maybe not really working, as it might be clear where the CC is going to be built by observing the units.
In this case, as i described the situation only 1 cav following my units was enough to observe my target location and block building by putting this cav on top of the site. I did not delete the CC, because of 2 reasons: 1. there was only this spot available, as all other closer areas were forest - rebuilding somewhere else would have led to find completely new area -> too much time. 2. i was afraid to loose resources by deletion and not beeing able to rebuild. (according to @real_tabasco_sauce - this would not been the case.
From the discussion can i take away, that at least some would agree, that blocking CC construction of an enemy is not a valid technique and unfair gameplay? So until this issue is solved technically a code of conduct for a host of a game might be just kick the player who blocks construction until construction of the CC (even on other location ) can be started.
Another solution as @AInur posted would be not to play Nomad mode.... but tbh... nomad mode on ambush map is really fun sometimes... i would miss it.