Not easily as arrays are fixed sizes in .NET. You need to copy the required array elements to a new array of the required length and re-assign the new arrray to your existing triangle array. This copying is likely to be fairly slow in Basic4ppc as I think your arrays are quite large.
However I have for a while now had an unposted version of my http://www.b4x.com/forum/additional-libraries/2565-byteconverter-library.html#post31631 which can manipulate arrays as byte buffers. I have just posted it so take a look at that. In the archive is BufferDemo.sbp which shows how to remove an entry from a multi-dimensional array. This is likely to be an order of magnitude, if not more, faster than copying individual array elements.