The Type itself that you've created cannot be removed from memory (or really more correctly from the AppDomain), but you can, in some situations, modify the columns of the corresponding EntityTable.
If you didn't define a primary key when creating the type, the actual EntityTable managed by the PM will always be recreated based on the latest query for the dynamic type.
If you did define a primary key, you can possibly still modify the EntityTable without harm, but it does require some care. How you do this modification may also depend on how you created the type, but here's one way:
// Get the table out of the PM
EntityTable tbl = mPm.GetTable(myDynamicEntityType);
// Modify the table
//Optionally reset the prototype table
You do this at your own risk, and it's doubtful that persistence operations on the type would work, but I haven't tried. If you're using a BOS you might also run into trouble with the schemas being out of sync between the client and server, but again I have tried it out.