No easy way no...
Edit the code

the modify and delete functions are located in the ModifyMessage.php sourcefile.
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Ok i have some code for you...
In ModifyMessage.php right below this line:
Quote$modifymessageplver="YaBB SE";
add this:
QuoteFunction DiabledFeature() {
fatal_error("This feature has been disabled");
}
Save & Close.
now in index.php, search for these 2 lines.
Quoteif ($action == 'modify') { include_once ("$sourcedir/ModifyMessage.php"); ModifyMessage(); }
else if ($action == 'modify2') { include_once ("$sourcedir/ModifyMessage.php"); ModifyMessage2(); }
coment them out.. put 2 // (forward slashes) in begining of their lines... now right below the lines... paste these in.
Quoteif ($action == 'modify') { include_once ("$sourcedir/ModifyMessage.php"); DisabledFeature(); }
else if ($action == 'modify2') { include_once ("$sourcedir/ModifyMessage.php"); DisabledFeature(); }
Save & Close
now replace the files on your server.
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Oh and the above code will completely diabled the features in there entirity, but its a quick fix.