htaccess hacked

Hello, this is the 4th time the htaccess of my wordpress website is “probably” hacked, suddenly a “500 Internal Server Error” appear.

I opened the htaccess file to find:
===========================
# BEGIN WordPre
ss
<IfModule mod_rewrite.c>
==========================

and sometimes i find in the htaccess around 100 duplicated content.

first i deactivated some plugins and repaired the htaccess, but the file is corrupted few days later.

I appreciate some help.