To remove arrow (or plus/minus) icon in Accordion Block, you can follow these.

#1. All Accordions
Use this code to Custom CSS box
div.accordion-icon-container {
display: none;
}

#2. All accordions (on One Page)
First, find Page ID. Use below tool.
In my example, we will have: #collection-66b1ea3b1c30c22054d0a383

Next, use CSS code like this
#collection-66b1ea3b1c30c22054d0a383 div.accordion-icon-container {
display: none;
}
#3. Specific Accordion
First, find Accordion Block ID. Use #2 Tool
Next, use CSS code like this
#block-yui_3_17_2_1_1722935850341_3420 div.accordion-icon-container {
display: none;
}
#4. Specific item in specific accordion block
You can find accordion block then use CSS code like this
#block-yui_3_17_2_1_1722935850341_3420 li:nth-child(2) div.accordion-icon-container {
display: none;

#5. Remove accordion icon on desktop only
@media screen and (min-width:768px) {
div.accordion-icon-container {
display: none;
}
}
#6. Mobile only
@media screen and (max-width:767px) {
div.accordion-icon-container {
display: none;
}