Open your web browser go to https://lightroom.adobe.com/ and login with your Adobe username and password.
(Or open from a synced collection in the working Lr Catalog)
Click the Cog wheel next to a Collection and you get the option to Delete it.
And as you have discovered you can only sync to mobile from one catalog.
Thanks for the info, the problem is that there is no cog wheel for the other collections. Only the current collection has a clog.
The ones i cannot delete are stretched when viewing.
I cannot find the link but I remember reading a similar question and the answer summary was "to delete the photos on the mobile device", but sorry I cannot be more helpful.
Maybe the solution would be to go to Lightroom.adobe.com and clean things out there.
I found my solution, you must go to wwww.lightroom.adobe.com in the upper left corner you see the Lr icon, click on that and down where it says "account info" click on that and you'll see "delete lightroom catalog" although there might or might not be, but it will ask if you want to delete "xxx" amount of photos and from there follow the directions.
thx everyone for the help