I'm not entirely sure it is possible. The assumption is probably that most people don't want some large blank space in the middle of their section. So when you delete a text area via trigger, the content is moved to consume the space it is presumed to leave behind. In theory, I suppose you could tie the behavior to whether the "can grow" attribute is defined on the text area, but I don't believe that it does.
There is currently not a method for you to turn off the white space management within a single section.
To achieve the desired functionality you will need to break out the conditional text area into a separate section. You will then be able add the new section to the form object and apply the the same trigger applied to the paragraph to the section. Additionally on the section following your conditional section you will need to set the Position options to "ABS - Absolute Page Position" for the Left Type & Top Type.
Hope this helps.
I'm going to guess that you have multiple overlapping text areas where you only expect one to trigger and the other to not. One easy solution is probably to use a Paragraph Selection on a multi-line text field instead of individual text areas that overlap. Instead of a text area, create a MLT field. Create a Paragraph Selection List (PSL) and add your prior text area content as paragraphs. Assign the triggers to paragraphs that you previously assigned to the individual text areas. Have the field use the PSL you created and assign the field with the ParagraphAssembly rule. That will evaluate your triggers during generation and the content you want will fill in and the text will “grow” instead of shrinking. There is an attribute that can be assigned to Paragraph Selection fields if you don't want the user to be able to edit the content - that is if you are kicking the transaction to WIP for local editing.