To put control in selecting unit of measure

Hello Experts,

Greetings,

I have scenario in which we are selling our products through different different distribution channel.We do sells our product in two diff "uom".The base uom is "ctn" and alt uom is "ea".The alternative uom has also been fixed accordingly.

Now the issue is how do we make control in system such a way that if person is supposed to make sales order in "ctn",he can not make the sales order in "Ea".

Any help would be highly appreciated.

Please advice.

Thanks & Best Rgds

Nitin