How to remove content appended to block in parent template?

I have three templates:

base.tpl

{block name="myBlock"}
    base
{/block}

child.tpl

{extends file="base.tpl"}
{block name="myBlock" append}
    child
{/block}

grandchild.tpl

{extends file="child.tpl"}
{block name="myBlock" append}
    grandchild
{/block}

The output when rendering grandchild.tpl is

base
child
grandchild

But I would like to skip the content added by child.tpl, so I would like to get this output:

base
grandchild

Problem is that this needs to be done without changing base.tpl and child.tpl (both come from a third party), I can't just extend base.tpl in grandchild.tpl, since there are more blocks with content in child.tpl I'd like to have in the result.

Solution

In this case you should simple change name of the block in base.tpl and grandchild.tpl and leave child.tpl not changed.

base.tpl

{block name="myBlock2"}
    base
{/block}

child.tpl

{extends file="base.tpl"}
{block name="myBlock" append}
    child
{/block}

grandchild.tpl

{extends file="child.tpl"}
{block name="myBlock2" append}
    grandchild
{/block}

Output is now:

base
grandchild

EDIT - after info from comment that base.tpl also could't be modified

It seems that the only thing you need to do is removing append from grandchild.tpl and leave 2 other files unchanged.

grandchild.tpl

{extends file="child.tpl"}
{block name="myBlock"}
    grandchild
{/block}

Output is:

base
grandchild

as you wanted