Here is the VickyWiki article on assimilation:
http://www.paradoxian.org/vickywiki/index.php/POP_Activity_Guide#Assimilation
Changing one of those major points listed will alter the assimilation cap.
Along these lines, something which
V The King found out was that if you alter the province.csv file to change what provinces are counted as being on the American continent, then you can alter where immigrants will go, and susbequently how large the size of the POP to be assimilated. If you don't want immigrants going different places then just change the American continent to Oceania or something (see the assimilation cap table for what sizes, etc... you are looking to be assimilated)??
In addition, remove the USA tag and replace it with a new tag with the same in-game name, etc... This would require a LOT of work, however, as all of the events files and AI files, etc... would need to be altered... so I wouldn't recommend that.