Yes you are free to remove the footer text.

In the Joomla 1.0 version:
Open the registrationpro.html.php file and this text is on line 608.

In the Joomla 1.5 version (1.5.7 and higher):
You will find the following lines 92 and 93 in the English language file in the languages folder of your Joomla 1.5 website.

EVENTS_FRONT_FOOTER_TXT_POWEREDBY = Powered by
EVENTS_FRONT_FOOTER_TXT = [Event Registration Pro]