Here is a quick, simple tutorial on how to have "Make votes public?" checked by default. This may eliminate the problem of members who do wish to have a public poll but are unable to change it after submitting it.

A small skin edit is required.

ACP > Look / Feel > Skin Manager > (choose skin) > Manage Templates / CSS > Post Screen > Poll Box >

Find...

<span class='ipsType_small' id='check_public_poll' data-tooltip="{$this->lang->words['poll_public_warning']}">
						<input type='checkbox' class='input_check' name='poll_view_voters' id='poll_view_voters' value='1' <if test="viewPollVoters:|:$data['poll_view_voters']">checked='checked'</if> />
						<label for='poll_view_voters'>{$this->lang->words['poll_fs_public']}</label>
					</span>

Change to...

<span class='ipsType_small' id='check_public_poll' data-tooltip="{$this->lang->words['poll_public_warning']}">
						<input type='checkbox' checked ='checked' class='input_check' name='poll_view_voters' id='poll_view_voters' value='1' <if test="viewPollVoters:|:$data['poll_view_voters']">checked='checked'</if> />
						<label for='poll_view_voters'>{$this->lang->words['poll_fs_public']}</label>
					</span>

Save the change. Repeat for other skins if required. Quick pic: