Mike Lagasse said:

We are trying to change the currency for one of our suppliers.

Sorry but once a vendor or customer is set up with a currency, the currency cannot be changed.  The rule is one currency for each vendor or customer account.  You will have to create a new vendor with a slightly different company name (eg. change the old one to LumenRadio Inc. - USD and stop using it) and create a new one in CAD.

Mike Lagasse said:

We also want to turn off the option to enter transactions using Foreign Currency

Again, sorry but once it is turned on you cannot turn it off.  You just have to stop using it.

This is why all the options are greyed out.

You say stop using it, what do you mean? How do we not use/use that option if it is turned on? 

In the real world, it's the same as that chair in the corner that either you are not comfortable sitting in, it's broken or the material is ripped, so you stop using it.  You can still see it's there if you look in the right direction, but it just doesn't get used.

In Sage 50, it's the same as not using the Project module, Inventory and/or Services or a bank account reconciliation after you close the account and/or change banks, or if you are using Premium or higher, not using Timeslip, or 100 Price lists.  The features are there and turned on but you just don't use them.  Some people use some, some don't.  Maybe you have 6 price lists but stopped creating more.  Maybe you only use 4 of those price lists now but they all exist.

So when you have created CAD vendors/customers for each of your USD vendors/customers, just stop using your USD vendors.  If you have a USD bank account, then after you close it and transfer the money to your CAD account, then you stop using it in Sage 50.  Therefore, you will not need the exchange rate box for any transaction and it should no longer show up on your data entry screens, because you are not referencing anything applicable to foreign currencies.