If I pay you to build a fence in my back yard, its my fence not yours. You used your design skills, your tools, your knowledge. If you are experienced or you gave me some super dazzling design that noone has ever seen, it is YOUR responsibility to charge me for that.
If 10 years later, a tree falls on the fence and I get my wife's cousin's dad who now does fences to fix it and he copies your exact design to make it match, so what.
This is why Identity and Branding companies get paid to do a logo and provide all the files, fonts and pantones. After its out the door who cares.
So if you do a design that you think is worthy, then you should charge for the design separately and give them the vector files. Obviously if they are going down the road later, they arent happy with you.
Now if you are in this conversation and you think picking a font and a color is worthy of a design.. then youre wrong. I give that type stuff away all day long and if my customer wants to go down the road for something ill shoot them the files, or if johnny
sign guy recognizes the font and recreates something for them, cool.
Youre damn sure that if a client comes in with orange text in impact font with a black star behind it and wants it reproduced, ill happily do it and take his money... if you think that is a "design" tighten up
All in all, trying to "lock" people in to using your service is garbage.. we ALL hate it with cell phone companies.. so why try and do it with your clients. If youre not charging for the "design" you had better build it into the cost of the first order, just incase you never see them again.