The space is due to the way some browsers parsing the html code.
I see this as the raw html
<div class="quote">Signed message: This 1 BTC is for sale by TheButterZone for $140<br>Signature: HOxlhwW83HZCEMw2DBGiqyf0dJFTgCtjvwpypPndsIzScc5RhD5xOC1PXaSveZIiL5pKG7bH3kZ47gW<span style="width: 0; margin: 0 -0.6ex 0 -1px;"> </span>ycbj0QUE=</div>
Notice the span injected in the middle of the signature. Not sure how you got part of the signature ended up inside a different div tag but different browsers are going to parse that differently. Chrome adds no spaces or line breaks but the html "standard" is more like a guideline and different browsers = different outcomes. Not sure the exact reason the forum's parser interprets a long string that way but as indicated above using code tag is a workaround.
Using code tags:
HOxlhwW83HZCEMw2DBGiqyf0dJFTgCtjvwpypPndsIzScc5RhD5xOC1PXaSveZIiL5pKG7bH3kZ47gWycbj0QUE=
Produces the following html:
<div class="code">HOxlhwW83HZCEMw2DBGiqyf0dJFTgCtjvwpypPndsIzScc5RhD5xOC1PXaSveZIiL5pKG7bH3kZ47gWycbj0QUE=</div>