Use the textbox event "On text changed".
When that triggers, put the action "Textbox: Set text to:"
Set the textbox text to:
replace(Textbox.Text, "+", "")[/code:234wu1iz].
If you want to remove [b]-[/b] as well, simply add
[code:234wu1iz]replace(Textbox.Text, "-", "")[/code:234wu1iz]
as well.
If you want to put all in one action, you can use regex for the Textbox text.
[code:234wu1iz]RegexReplace(Textbox.Text, "\+|\-", "", "")[/code:234wu1iz]