What you could do is edit the template calendar_edit and change the timezone and dst fields to be hidden fields with the values that you want. For instance like this:
<vb:comment>
<label for="range_sel_timezoneoffset">{vb:rawphrase time_zone}:</label>
<select name="timezoneoffset[range]" class="primary" id="range_sel_timezoneoffset" title="{vb:rawphrase time_zone}" tabindex="1">
{vb:raw timezoneoptions}
</select>
<p class="singledescription">{vb:rawphrase description_enter_timezone}</p>
<label for="range_cb_dst"><input type="checkbox" name="dst[range]" id="range_cb_dst" value="1" {vb:raw dstchecked} tabindex="1" /> {vb:rawphrase maintain_same_time}</label>
<p class="singledescription">{vb:rawphrase description_ignore_dst}</p>
</vb:comment>
<input type="hidden" name="timezoneoffset[range]" value="5" />
<input type="hidden" name="dst[range]" id="range_cb_dst" value="0" />
This only shows part of the template, of course, and the parts I added are in red. You'll want to change the value="5" to the timezone offset you want, in hours, and you could change the range_cb_dst value to 1 if you want to ignore DST. (BTW, I haven't actually tried this).