how to disable question title autocomplete?

You can add the following code to the style.css file: 

.ui-autocomplete { display: none; }

You can use the following code to hide the search autocomplete:
.dwqa-search .dwqa-autocomplete {
display: none;
}
If you want to use the autocomplete and fix the search box, you can use the following code: 
.dwqa-search .dwqa-autocomplete {
top: 40px !important;
}