How would I stop players from inputting characters into an editable multi-line text box when the text box is full?

Not sure if you’ve already seen this, but here is a similar thread with several solutions.

Also, it looks like in your blueprint code you are just changing a text variable, but not actually setting that variable to the textbox object. Use the “SetText” function, from your textbox object to set the text of that textbox. You can see examples of this function in the link above.